Symptoms
Unable to choose Roundcube skin in Roundcube UI at Settings > Preferences > User Interface after Plesk update.
Cause
Expected behavior.
Roundcube is open source software and on their most recent version (>=1.6.0) the skin menu has been removed and the package is shipped by default with the modern Elastic interface skin. The old skin Larry is not used anymore. This change has not been made by Plesk but rather by the software vendor.
Resolution
As a workaround to have any older skin set, manually customize the default skin by following below instructions:
How to add a custom skin for Roundcube Webmail in Plesk?
Note: Any manual skin changes may be overwritten by Roundcube package updates.
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