Oops... Rspamd DKIM signing does not work for subdomains.
Initially I thought it was a problem with selection of proper key because rejected messages included base domain in DKIM-Signature field, not subdomain. I tried to change the selector. This did not help. But when I removed base domain from the rspamd configuration completely and left subdomain only, it stopped signing messages at all.
I need some replacement and I'll investigate this soon, when I'll move my mail services to a different server.
To make the world better the best replacement would be an implementation of mail-my-own idea to ease deploying mail service for your domain anywhere, even on your smartphone.
But I have no time for this yet.