I followed installation of kolab using https://kb.kolab.org/2023/03/install-kolab-16-on-alma-linux-9 and the roundcubemail/errors.log file shows:

[20-Apr-2024 14:54:54,803351 +0200]: <o8tmlbgi> PHP Error: required kolabformat module not found (GET /webmail/xxxxxxxxxxxxxxxx/?_task=mail&_mbox=INBOX)

It turned out that the kolab package did not install php-kolabformat on Alma Linux 9. After installing dnf install php-kolabformat this issue was resolved.
I would propose to include php-kolabformat into the kolab package.

# rpm -qv pykolab
pykolab-0.9.0.6-1.4.el9.kolab_16.noarch