The fix-cache-key is used when you move a site (either to another directory, or to a new server). Its purpose is to re-write the correct PATH to the cache folder (a database reference). It will not solve the problem(s) you are reporting.

Fix-cache-key can be done manually by making appropriate edits in CONFIGURATION>>>SESSIONS and CONFIGURATION>>>LOGGING. However, some people do not know the server paths, so it is usually safer for them to run the fix-cache-key script.