- Plesk Onyx for Linux
- Webmail type of the main domain of a subscription or webspace is set to one value, for example: 'Horde'
- An additional domain under the subscription/webspace, webmail type is different, for example: 'None' or 'Roundcube'.
- After securing the main domain with Let's Encrypt, webmail on the additional domain is set to the same value as on the mail domain ('Horde')
- The same issue might occur during the automatic renewal of the SSL certificate
- #EXTLETSENC-603 "The webmail client of an add-on domain is no longer changed to the client of the main domain (or even disabled if webmail was disabled for the main domain) when any of the following actions are done to the SSL/TLS certificate of the main domain: issuing, manual or automatic renewal, enabling “Keep websites secured”, or unassigning."
If update is not possible for some reason you may try the following
Until a fix is available, use the same webmail type on all domains under a subscription/webspace that are using Let's Encrypt in:
Plesk > example.com > Mail Settings > Webmail