OK, so what is your question?

I mean it should not happen, because I try to post transactions to current period, which is no relationship to P10 of last fiscal year. Why did unlock P10 of last fiscal year help solve the problem?

Because that transaction affected P10 of last year!  Check your work, there is no bug.

Thank you Jay! But I checked the entries, nothing is related to P10, we posted only transaction for current period.

My answer doesn't change.

I've only seen this in old versions (around 5.4/5.5 ish) where you had to unlock period 1 of the prior year to post to period 1 of the current year.  I never figured out why and nothing actually did post to that year.  It was an odd bug that only cropped up a handful of times and not for all clients.

Turn on rvSpy if you're interested in digging it up.

Thank you Django! I would let it go for now, as it happened only once.