I encountered a problem in one of the sites with a J51-template I upgraded to Joomla 4. The template is Revolve. The module mod_j51gridgallery gives only on the homepage this two errors:
"Deprecated: preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in /home/webtester/domains/http://alstest.nl/public_html/wegmetk/modules/mod_j51gridgallery/tmpl/convert_rgb.php on line 14"
and
"Warning: Trying to access array offset on value of type bool in /home/webtester/domains/http://alstest.nl/public_html/wegmetk/modules/mod_j51gridgallery/tmpl/default.php on line 116" (and the same for lines 117 and 118)
(also the css for this module is disturbed, but I think I can manage; however this problem disappears after fixing the errors)
I already made a superadmin login for you.