Ubuntu 26.04 was recently released. The webmail package roundcube did not get an update and is still at version 1.6.11 released in June 2025, that was already included in Ubuntu 25.10.
This is a pitty as te PHP version in 26.04 was updated from 8.4 to 8.5 compared to Ubuntu’s previous release 25.10. And with that version update came the introduction of an array_first function for PHP. Up to version 1.6.11 roundcube defined such a function within it’s own code which now conflicts with the PHP build-in function. The roundcube developers already fixed this problem in July 2025 and the fix got released with version 1.6.12 in December 2025.
As this fixed version did not make it into the current Ubuntu release, we now have a roundcube version in Ubuntu 26.04 that fails to run with PHP 8.5.
What you could do now is to dump the Ubuntu roundcube package and install a roundcube version including the fix by directly downloading from roundcube’s website. This would give you the opportunity to upgrade to the just released version 1.7.0.
Another option would be todowngrade PHP to 8.4 (probably not recommended).
For me the easiest way seemed to just apply the fix from Commit 060fc95 to program/lib/Roundcube/bootstrap.php. It’s just a few lines of code and afterwards the patched roundcube 1.6.11 is up and running again and not conflicting with PHP 8.5.