OK, I’ll take one more shot at this, but this is starting to approach IBM PMR status (although this is pretty good debugging information for other people that might see something like this)…
See this excellent @jgstew post on how to get PMR help: How to ask for IBM product help: PMRs, RFEs, and more
FIRST, let’s get logs to see if there’s any clue as to what’s happening. Try logging in a couple of times and getting to the 403 error, then immediately copy everything in your WebUI logs directory (by default it’s C:\Program Files (x86)\BigFix Enterprise\BES Server\WebUI\logs
Next, I would stop the root server service, then delete everything in the WebUI directory. Start the root server service again and wait a while for all the WebUI services to come online, the ETL process to finish caching things, and try logging in afterwards. This is a safe operation to do, you’re essentially just deleting all the cached WebUI things. WebUI will restore itself immediately afterwards.
If after restarting you still aren’t able to login properly as user “Administrator”, let’s try taking this to a PMR and figuring it out there.