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Lets suppose A casts a pain spell on B on the first round successfully making his spellcasting test. In the second round A fails his effect test to maintain the spell. Can B act in the second turn?

Yes.
(Didn't we just have a large Pain thread around here somewhere, or discussed this in another thread? Anybody who finds it can maybe post a link.)

i think it was on the old forums maybe ? the one that was also mixed with initiative problems tied to the low ini of nethermancers and such

That's the one. Was that on the old forums?

You'll do everything as normal (B announces actions, rolls initiative, etc.) and only once A succeeds, B can't do anything any more. The spell alone has no effect on initiative or changing actions.
Note A also makes his Effect Test on his Initiative, so B might get to do all his stuff before Pain can even take effect. That's the initiative issue galafrone was talking about. If B is faster, the best thing Pain does for A is preventing B from taking defensive Action (Avoid Blow etc.). Also note A can opt out of making the Effect Test (because it uses his Standard Action as a concentration type spell), and can resume making the effect test in another round when he is faster.

omg!!! until now we played it so B cannot take any actions until A fails an effect test. it was so powerful many thanks!

do i understand correctly that if A fails once, that doesnt end the spell, and next round A can make an effect test again?

Yes, as long as A uses their standard action to do so.