So...

1. Weblinks won't install from the Install from Web tab in com_installer in PHP8. It will install from package file.

2. Create category. Then create some weblinks.

3. Create Menu Items for any type of Weblinks menu item type and they throw the errors Harald mentioned above.

i can confirm

Weblinks won't install from the Install from Web tab in com_installer in PHP8. It will install from package file.

nor from install from URL

with
Error
Unable to detect manifest file.

does other extensions work? I test 2 other extensions without problems so I'm not sure if it's weblinks or joomla

i've installed Akeeba Backup Core 7.4.0.1 (2020-11-18), and despite

PHP version 8.0 is newer than this software supports

seems to work

I installed two other extensions (Akeeba and JCE) via the Install from Web tab without issues. I agree with @alikon that I too got the message with Akeeba that 8.0 is newer than the software supports. JCE installs and displays but some functionality doesn't work presumably because it isn't ready for 8.

So I think the issue is with Weblinks and not with Install from Web.

@HLeithner

Here a Call Stack from German forum: https://forum.joomla.de/thread/13254-fehlermeldung-bei-aufrufen-von-kategorien-bei-addon-weblinks/?postID=83873#post83873

It appears Weblinks is trying to use the property "item", which is not defined for WeblinksViewCategory. I updated a few lines to change $this->item to $item, as they're contained in a for loop that iterates across all the links in the category. Also had to put in an If test to get around a $this->item reference outside of the loop. My only use case is displaying the links in a category, so I can't promise this is an absolute fix for all use cases. Apply these changes at your own risk, and make a backup copy of modified files so you can fall back to standard if it breaks other things.

In components/com_weblinks/views/category/tmpl/default_items.php I made the following changes:

Line 17:

Old:
$params = &$this->item->params;
New:
$params = null;
if (property_exists($this,"item")){ $params = &$this->item->params; }

Line 131-132 (Original, 132-133 after previous change):

Old:
<?php $this->item->tagLayout = new JLayoutFile('joomla.content.tags'); ?>
<?php echo $this->item->tagLayout->render($tagsData); ?>
New:
<?php $item->tagLayout = new JLayoutFile('joomla.content.tags'); ?>
<?php echo $item->tagLayout->render($tagsData); ?>

With the recent commits, the two reported issues are fixed. So I'm closing this.

I get this on a couple of Joomla 3.10.11 main menu items now ( just a couple, everything else is fine)... Just testing migration to PHP8

( Error: 0 - Attempt to modify property "params" on null )

Confirming: Since my IP upgraded to PHP8.0 Weblinks reports error

"#0 Attempt to assign property "tagLayout" on null"