Some background information:
The BucketLockService is used to guarantee uniqueness or as a kind of semaphore. It is used all over the Quality Manager code. For example, in this context of saving a test case, it can be used to guarantee that the name of the test case is unique if that project setting is in place. There is a possibility that the lock will cause a false positive, and so the transaction might be retried. There is a limit on the number of retries, and that was exceeded here. It is certainly unusual to have multiple retries unless the server is under heavy load.
It is hard to determine exactly what might have caused the issue without more information. There was probably a stacktrace returned to the client browser, and that could be extremely valuable in determining the root cause. Unfortunately you would need to be using developer tools in the browser such as the Firefox/Firebug console to see that.

I am able to reproduce the limit in QM 602 iFix004 environment. The limit for test case category appears to be 50 per QM project areas. The number includes live + trash +  permanently deleted category. In 602 the error message is better. The message you get while creating a new category not whilst trying to refresh or a save test case.

"You have reached the limit of 50 categories"

If you need new category either consider a new QM project area or associate them as "add value" to an existing category.