flohmarkt.conf.example there is a line
ExternalURL = http://example.org
which I interpreted as put your domain here as http. This is valid for test running the software and connecting directly to its http server.
To run it on the internet I added a reverse proxy forcing the use of https. For http requests the reverse proxy answer with a redirect 301 to the same location via https.
Mastodon seems not to follow these redirects when posting messages for flohmarkt users.
But if the
ExternalURL is set to an http address this address is propagated via webfinger to flohmarkts communication partners which than try to use http and do probably not follow redirects for POSTing to flohmarkts /inbox.
Short: no message reception from Mastodon instances if flohmarkt runs behind a reverse proxy that wraps http into https if not https in