Good news, everything started to work ok after replacing 8vac with 12vac transformer. No more moteino reboots on ring.
However it didn't detect 1st button ring after startup, but all the further ring events were normally detected. I'll do some more ring tests. but for now everything looks good.
Thanks for your help and suggestions!
EDIT: when I ring with button, ring event is not always detected. maybe if I push the button quickly or something... otherwise moteino works fine, no more reboots