You have installed patch 18.104.22.168 and find that you are unable to save changes to tickets in service desk.
You receive the message
Method not found: 'System.String ServiceDesk.Common.
A file that should have been included in the 22.214.171.124 patch was not there
WORKAROUND / SOLUTION
Please see this KB article for instructions - https://kaseya.zendesk.com/entries/96438148-Email-reader-not-working-after-install-of-9-1-0-11
Although this mentions the email reader, the instructions are the same.