Symptoms
Extensions is not available in Plesk with:
Internal server error Retry
The following message if found in /var/log/plesk/panel.log:
Could not resolve host: ext.plesk.com
Cause
Corrupted Plesk extension catalog cache.
Resolution
Connect to the server via SSH;
Remove cache files:
#rm -rf /usr/local/psa/var/cache/https*
Create Plesk database backup
Remove cache records from Plesk database:
#plesk db "delete from PersistentCache where events like '%catalog%'"
Restart Plesk service:
#service sw-engine restart