The issue is likely to be with the linked action you use for opening an existing case.
The following should be done to resolve:
Open Proclaim screen designer
Go to the linked action screen for the case type affected
Check the name of the linked action against the option Run this linked action when an existing case is opened
Make a note of the linked action name
Open Workflow maintenance
Choose the relevant case type
Open the linked action tab
Find the linked action you noted the name of above and edit it
Check the math's for the screen name in the error message
Make any changes required to amend the name or remove the entry for this screen
Log out and back into proclaim and text
Test
If you do not know how to access or do the above then please raise a new case online and reference the title of this article.