I cannot click "Change password" here. This is not fixed by enabling or disabling (any) other plugins. Nothing in Roundcube error logs, jqueryui plugin is activated (maybe not properly?).
Verify you have installed ISPConfig Roundcube Plugins correctly and configured them following the guide. The plugin author has some troubleshooting tips on the documents site.
Have you used this guide? https://www.howtoforge.com/install-ispconfig-3-roundcube-plugins-on-debian-10/