Change in projectkey.pattern results in "The project you are trying to view does not exist"

After changing the jira.projectkey.pattern from ([A-Z][A-Z]) to ([A-Z][A-Z][A-Z0-9]) viewing an issue (../browse/<issue-id>) doesn't work any more an the following error message is presented:

"The project you are trying to view does not exist. Try browsing for projects."

The project and issue have previously been created in jira 3.7.3

The same configuration works in jira 3.7.3.

I tried to reproduce it with atlassian-jira-enterprise-3.9-standalone package but it's possible to create the project. JIRA complains that the project key "TEST2" is not valid for the projectkey.pattern=([A-Z][A-Z][A-Z0-9]+)

I guess this is somehow connected.