When I issue search command :Ack a it produces no output. I can see that rg is running in background with pgrep -a rg.

1272517 rg --vimgrep --no-heading a

When I try to search again it fails with error that it cannot cancel job.

Error detected while processing function ferret#private#ack[11]..ferret#private#nvim#search[1]..ferret#pri
vate#nvim#cancel:
line    3:
E474: Invalid argument

Looks like the cause of the problem is a change in rg BurntSushi/ripgrep#1892