Wow, for max_execution_time, that’s quite a while — most sites we see have 30-60 seconds instead. I’m not quite sure why a shorter value like 300 would hang without a more specific message.
But the “Scan process ended after forking” message without other messages the end of a scan usually means that the site can’t connect back to itself to start the next scan stage. On the new Diagnostics page, you can turn on both “Disable config caching” and “Start all scans remotely”, to have our server try to reach your server instead, which may help. You can also turn on “Enable debugging mode (increases database load)” temporarily if you haven’t already — it gives more details about the scan, which may help.
The “Fetching stored cronkey for comparison” message suggests a database issue or possibly having two scans started at once, since the cron key normally shouldn’t disappear on its own. (Running two scans should be nearly impossible, but I managed to cause it a while back during testing another issue.) It might just be a side-effect of testing repeated scans with a very long execution time.
Let me know if the “Start all scans remotely” option helps, or if you can see any new details in the scan results.
-Matt R