Sophos UTM / User Portal: Works about once per day (locally defined users, not AD)

Question

I am able to authenticate about once per day on the user portal with one user on one computer. If I do it again with another user's credentials, I get the rather meaningless "user/password wrong or policy blocking" - popup. I do it again the next day, I get again one shot. 
 With about 50 users who I would like to advise to look into their spam reports online... that's a tiny bit of a complete road block. 
 Some part on the Sophos UTM seems to cache a part of the login configuration, and simply rejects anything but the first remoteIP:userLogin pair. 
 a) Is there a way to debug this behaviour, to validate my theory? b) Is there a way to circumvent this behaviour, e.g. by adding an undocumented parameter that skips the cache? (?no_cache=1 does NOT help.) c) Is there a way to fix this behaviour by means of a hidden configuration setting? (I don't mind logging in as root and set some obscure flag, as long as things work afterwards...)

apijnappels · Accepted Answer

Under Web Protection -> Filtering options -> Misc there is a caching section. 
 I don't know whether or not this could be the culprit, since I have always had this off. I don't think there's real benefit in having it on especially with high-bandwidth connections. If your's is on you can at least try to clear the cache or also disable to function and see if this changes things.