Tested on a clean Joomla 5.4.5 → 6.1.0 upgrade with a multilingual setup.

Steps performed:

• Installed multiple languages (French, German, Spanish)
• Verified language update sites are enabled and pointing to translationlist_6.xml
• Rebuilt update sites and cleared cache
• Checked updates multiple times

Result:
No error appears. Updates are fetched correctly and the system behaves as expected.

Conclusion:
Unable to reproduce the issue in this environment. It may be related to specific server configurations, cached data, or legacy database entries after migration.
@Valentyn-M Could You help me to reproduce the bug by giving some hints.

Thanks for testing.

Important clarification: this is probably not a Joomla 6.1 migration-only issue.

I can confirm that the same red error was already present on my site before the upgrade, while it was still running Joomla 5.4.x. After upgrading to Joomla 6.1.0, the same behaviour remained.

So the issue seems to be older and not strictly caused by the 5.4 -> 6.1 migration itself.

Another important hint:
the problem reproduces not only on the production server, but also on a local restored copy of the same site.
That suggests it is likely related to the specific site data / multilingual setup / update-site state, not only to the hosting environment.

What I already tested:

• rebuilding update sites
• clearing Joomla cache
• manually reinstalling/updating language packs
• enabling/disabling the official language update sites

Result:

• when the official language update sites are enabled, Extensions -> Update shows the red error
• when those language update sites are disabled, the red error disappears

So a clean Joomla install may not reproduce it, but an existing multilingual site with older data does.

If useful, I can provide more exact details from the affected site state and test additional scenarios you suggest.

Yes please.

What exact additional information would be most useful for you to reproduce this issue?

For example, I can share:

• specific screenshots,
• the exact language/update-site configuration,
• database rows related to update sites,
• debug output,
• or results of any particular checks you would like me to run.

Please let me know what would help most.

Thanks! To help reproduce this, could you please share:

• The debug error output (with Debug System enabled)
• The exact language update site URL(s)
• Whether the issue happens for all languages or only specific ones

This should help identify the failing condition.

Thanks. Here are the requested details.

1. Debug error output (with Debug System enabled)
   I enabled Debug System and reproduced the issue again on the affected site.
   In browser DevTools -> Network, the Extensions -> Update page request returns 200, but the response still contains the Joomla red error message: "Could not fetch update information, enable system debug mode for further information."
   I also checked the Joomla Debug Bar / Queries tab, but it does not show an obvious SQL exception. I am attaching screenshots of the page, the Network response, and the debug output.

2. Exact language update site URL(s)
   The official language update sites on this affected site point to: https://update.joomla.org/language/translationlist_6.xml
   This is shown under: System -> Update Sites -> Accredited Joomla! Translations

3. Whether it happens for all languages or only specific ones
   On this affected site, the issue is not limited to one specific language.
   It appears whenever the official Accredited Joomla! Translations update-site group is enabled.
   In my current Update Sites list, this affects the official language update entries present there:
   Dutch, English, French, German, Polish, Russian, Spanish, and Ukrainian.
   Swedish is installed on the site, but it does not appear there as a separate official language update-site entry.

response-update-page.txt

swedish does not appear because the manfiest file is missing the update server information in the file administrator\manifests\packages\pkg_sv-SE.xml

	<updateservers>
		<server type="collection" priority="1" name="Accredited Joomla! Translations">https://update.joomla.org/language/translationlist_6.xml</server>
	</updateservers>

@tecpromotion can look at fixing that bug

However after installing all the languages you have installed I cannot replicate the reported problem when checking for updates on the other languages. I do see in your screenshot that you are getting a 303 redirect to something called speculationrules - I am guessing but maybe thats some cdn thing

Thanks, I checked that part more carefully.

The earlier speculation entry in DevTools was misleading because I had Preserve log enabled in Network.
After disabling Preserve log and testing again, that speculation request disappears from the fresh request list.

So the remaining 303 is not a redirect to speculationrules.
It is the Joomla request:

index.php?option=com_installer&task=update.find...

which returns 303, and then redirects back to:

index.php?option=com_installer&view=update

with 200.

The red Joomla error still appears in this clean test.

I also tested with Cloudflare Speed Brain disabled, and the problem still remains.
So at this point it does not look like the speculation / Cloudflare Speed Brain request is the cause of the Joomla update error.

Current language update site URL for all installed language packages is:
https://update.joomla.org/language/translationlist_6.xml

The issue happens regardless of which of the installed language update sites is enabled.
If at least one official language update site is enabled, the red error appears.
If all official language update sites are disabled, the red error disappears.

If useful, I can provide a fresh screenshot from Network with Preserve log disabled, showing only the real requests from the current test.

Thanks, I checked that part more carefully.

The earlier speculation entry in DevTools was misleading because I had Preserve log enabled in Network.
After disabling Preserve log and testing again, that speculation request disappears from the fresh request list.

So the remaining 303 is not a redirect to speculationrules.
It is the Joomla request:

index.php?option=com_installer&task=update.find...

which returns 303, and then redirects back to:

index.php?option=com_installer&view=update

with 200.

The red Joomla error still appears in this clean test.

I also tested with Cloudflare Speed Brain disabled, and the problem still remains.
So at this point it does not look like the speculation / Cloudflare Speed Brain request is the cause of the Joomla update error.

Current language update site URL for all installed language packages is:
https://update.joomla.org/language/translationlist_6.xml

The issue happens regardless of which of the installed language update sites is enabled.
If at least one official language update site is enabled, the red error appears.
If all official language update sites are disabled, the red error disappears.