Hi @girish, sorry to resurrect an 18-month old thread but I thought it might be better than making a brand new thread.
I'm experiencing the same issue with the latest version of Dokuwiki. I think it might be an issue with the bundled PHP version?
Here's how the problem manifests using the Cloudron-packaged Dokuwiki:
That results in this error logs:
Dec 30 14:33:18 [Mon Dec 30 21:33:18.755569 2024] [php:error] [pid 79] [client 172.18.0.1:32850] PHP Fatal error: Uncaught Error: Undefined constant "DOKU_INC" in /app/data/templates/bootstrap3/css.php:84\nStack trace:\n#0 {main}\n thrown in /app/data/templates/bootstrap3/css.php on line 84, referer: https://[REMOVED]/lib/exe/css.php?t=bootstrap3&tseed=a3c69f60143e699ab115489e1785128d
However, when I install DokuWiki by hand using the LAMP stack app from Cloudron, the error doesn't present itself.
The PHP versions seem to be off: 8.3.14 (LAMP, working) vs 8.1.2 (DokuWiki package, not working).
The LAMP:
The DokuWiki:
For testing purposes, would it help if I somehow manually updated the Cloudron-packaged Dokuwiki's PHP version to see if that changes things? I'd have to research doing that, but I'm happy to give it a try for learning's sake. 
Thanks for all you do!