Switch not accessible via web interface after OS update

[faflu]

I've got a stack consisting of 4 hp procurve switches:
2510G - commander
2x 2510-48, 2610-48 - members,
After OS upgrade to rel 11.57 of both 2510 switches, they became inaccessible in a web browser. When I try to select them - 'Bad request: Your browser sent a request that this server could not understand' - error appears.
I've tried to detach and rejoin them as stack members but without success.
Perhaps there are some steps to be taken after an upgrade, but are not stated anywhere in manuals (or I haven't foud them)?