Please help us help you and fill where relevant:
Your LimeSurvey version: 5.6.65+240521
Own server or LimeSurvey hosting: Own server
Survey theme/template: N/A
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Upgrading to 6.5.9+240521 causes all default survey templates to give a compatibility error, and prevents accessing surveys due to thrown exception "Could not find a working installed template".

Same here. Just updated to the latest version (with comfort update) and now nothing is working. Also, the suggested documentation is fairly lacking: www.limesurvey.org/manual/Extension_compatibility#Survey_themes

same here after doing comfort update to newest version (Release 6.5.9+240521) - i guess the reason for the error is a typo - it reads 5.6.65 in my admin backend, should be 6.5.9 - as i have no idea where the version number is stored i did a quick workaround and set compatilbilty in theme config.xml to 5 - but i guess it is better to wait for 6.5.9.1 with corrected numbers

Hello,

we have now fixed the ComfortUpdate package.

It will properly upgrade now. If you already did, you will need to restore a backup and do it again.

We are terribly sorry for the problem - apparently there was a conflict while files for the update packages were prepared, causing this problem.
We will investigate and make sure this won't happen, again.

Hi Carsten,
thanks for the reply.
I followed your instructions and still things seem not to be working: i.e. I ran ComfortUpdate yesterday and today I restored a backup from two days ago. Still, the exception message saying that "there is no working template installed" keeps appearing.
Any suggestions?
All the best!

You would need to restore the database too, or at least the table 'templates'.

Ok, that worked. Thanks!
Fingers crossed this doesn't happen again
Cheers,