If one changes projectkey.pattern in a way that the existing project keys don't match any longer there regex pattern, JIRA reports the following error:

ERROR
The project you are trying to view does not exist. Try browsing for projects.
If you think this message is wrong, please consult your administrators about getting the necessary permissions. 

The error is thrown if user try to access any issue from the problematic projects.

Most commonly, this issue is observed in two cases:

1. JIRA upgrade - where in the old instance projectkey.pattern was customised and the new instance has the default value.
2. Customisation of the parameter - admin user changes the value, but doesn't confirm that all project keys match the pattern in the regex.

In either of these cases users get confused and believe that either issue/project is deleted or there is indexing issue.

The "simplest" resolution would be to modify the error message to be more context specific. Alternatively, in a more complex solution some extra validation would be performed on the used project keys against the previous/default regex value and its current value. Finally, the feedback from this validation would be outputted into UI and log file.