I just tested on a random fic using the latest beta - getting a challenge error now, though nothing comes up in browser. I did an update of a pretty bulky fic yesterday though (it went through), so it might be possible my IP got flagged—unfortunately currently physically unable to reset the modem to check.
Googling the Cloudflare IUAM possibility error brings up a few bug reports from a (legally dubious, so no links) project where the response was "it's a site issue, use another one."
The IUAM error (which I'm not getting) seems related to the under-attack mode being enabled again, but I'm not seeing any evidence of it being used in the browser (no browser check, and the specific cookies related to the mode aren't present).
It could be possible IUAM's enabled only for some countries/IP ranges, or perhaps something in the way FFF/Cloudscraper works managed to trip a firewall rule. Or even a misconfiguration - not like the ffnet admins seem to have any idea what they're doing.
Fun thing to do: go to Twitter, search for @fictionpress, and
sort by latest.