I use Catznip 14.0.2 primarily, but have seen this on Firestorm as well, most recently with Beta 7.1.12.77526. My initial attempt to log in will fail & then i will immediately try again and that attempt will get me logged in. This has been going on for months.
I do have a large inventory (over 500k items & over 80 folders).
Related - I find there are some sims that I simply cannot log into at all, so wind up just logging in from my home sim.