I have been using the Broken Link Checker plugin on many websites for many years and apart from having to correct some deprecated code when using PHP 7.1, it has always performed very well with few issues.
I delayed updating to from version 1.11.21 to version 2.1 until I would have time to check the details of the new cloud version. I decided to stay with the Local (old) version and updated 7 websites and installed new on 2 more websites today. I went through the settings, but made few, if any changes. After updating, I hit the Nuclear option button on each. Of the 7 original installations, only 2 sites are still working.
The sites where the BLC is working display:
No broken links found. No URLs in the work queue. Detected x unique URLs in y links.
This is typical of what I have seen over the years.
The sites (5 updated, 2 new installs), where the BLC is not working display:
No broken links found. No URLs in the work queue. Searching your blog for links...
This has not changed after some hours and seems to indicate that the scan is not starting.
All websites are on the same hosting, but may be on different servers (all LiteSpeed). The two sites where the BLC is working are on PHP 7.4.33. The remainder are on PHP 8.0.28, but configuring other sites to PHP 7.4.33 had no effect. Rolling back to BLC version 1.11.21 did not fix the problem either.
Deleting version 2.1 and reinstalling it has fixed the first of the non-working installations and I am now repeating with the others. After repeating the procedure on the other non-working installations it was successful except for one site, which was running WP OPcache. After deactivating this plugin, BLC started working.
Note that each installation ws made without changing any settings but not yet observing it starting. After clicking the Nuclear Option all started.