I'm on 3.3.4 too. Waiting for 3.3.6 to be out and hoping to be stable.
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I did testing on 3.3.6 and didn't experience that issue. It makes me wonder if the bug that caused the 3.3.7 release is due to an interaction with an extension. Anyway, forget what I said about phpBB 3.3.6 -- if there is a new release, 3.3.6 should be avoided.
At this point it would be worth waiting a few days before updating to 3.3.7.
Seems like every release there is a major issue that needs rectifying, I only updated last night too!
I didn't experience this issue either but what a pain having to rush to check and upgrade again this morning.