Why it is disabled after the upgrade:
Release notes 7 — NethServer 7 Final
To prevent errors during Nextcloud upgrades, thetheming
have been disabled. After each upgrade, both applications should be manually updated and re-enabled by accessing Nextcloud administration interface.
On Nextcloud, did the mail app update successfully? After the mail app update did you re-enabled it?
Not related to your problem, but you might also want to check:
New installations of Nextcloud honor the StartTLS setting of the Active Directory accounts provider. As old installations ignore that setting and always send clear-text passwords, it is recommended to upgrade them to the new behavior. Make sure the remote AD accounts provider supports StartTLS, then run the following commands
config setprop nextcloud HonorAdStartTls enabled signal-event nethserver-sssd-save