Found the reason why db acc had stopped working: I had removed async_workers parameter. Looks like acc module does not insert anything if db_insert_mode=2 and there is no async workers. From README, I get the impression that it should be doing standard insert in that case:
6.28. db_insert_mode (integer)
If set to 2, async insert is used if the db driver module has support for it and if async_workers core parameter value is greater than 0. If not, then standard INSERT is used.
-- Juha