@alikon could you confirm this?

Correct command should be :
UPDATE #__users SET params = REPLACE(params, '"helpsite":"https:\/\/help.joomla.fr\/index.php?option=com_help&keyref=Help{major}{minor}:{keyref}"', '"helpsite":""');

The new value have to be replaced for the next release and as workaround.

French websites are going down one by one.....

TBH, you would've been better off leaving the user params alone versus trying to remove it. With the param gone from the UI, and the code not reading it, it would eventually cycle itself out as preferences get updated or just sit there and take up 50 bytes in a database field.

Now you've got corrupt JSON to deal with.

My suggestion to fix this would be.

UPDATE #__users SET params = REPLACE(params, '",,"', '","');

Would this work without side effects?

I tried
UPDATE #__users SET params = REPLACE(params, ',,', ',"helpsite":"",');

It recreates helpsite param.

replacing only ,, could lead to modify data of other fields

I tried
UPDATE #__users SET params = REPLACE(params, ',,', ',"helpsite":"",');

It recreates helpsite param.

There is no need to recreate the helpsite parameter, it is not used anymore in the code. It is only necessary to make Json valid again, i.e. remove the ,,, but in a safe way.

I created a PR to fix this #25177

normally the helpsite is between 2 other parameters I don't think that it's possible that the ,, are at the beginning or the end...

@HLeithner I think the same, but am not 100% sure. But I guess you wanted to comment that in your PR.

@pmleconte please test #25177

@pmleconte Fixed with new release 3.9.8.

@pmleconte And thanks for reporting it so fast.

closing as this has been patched by 3.9.8 thanks everyone involved.