Extensions is not available in Plesk with:
Internal server error Retry
The following message if found in
Could not resolve host: ext.plesk.com
It might be not possible to retrieve additional Plesk license in Tools & Settings > License Management with the following error:
cURL cannot communicate with license server https://ka.plesk.com/xmlrpc (): Couldn't resolve host name(6)
Corrupted Plesk extension catalog cache.
Warning: Before applying this solution to a retrieving license error, restart
sw-engine first and check if the issue is resolved. Only try to remove the cache if that doesn't work. Confirmation that the license issue is related to the cache is necessary because it looks strange.
Note: Update the article accordingly after checking the warning above.