Using Easytag, but...
Have configured the CDDB search to use gnudb.org, but searching for an Album, using "Search using selected files", (yes, it finds a bunch of albums), then clicking on an album to get the entries, I get "the server returned a bad response 'HTTP/1.1 404 Not Found'".
This issue seems to have appeared recently, previously it was working.
Thanks.

Can we get some more details about your situation?
What exact messages, what albums, possible log content...

All I can say is that I use abcde sometimes and had to change the CDDBURL:

You never mentioned what Linux distribution you're using and whether EasyTag had recently been updated. That might be worth looking into. If you have log files that show when updates have taken place, perhaps you can correlate the appearance of this error with a EasyTag update.

I can see how this error could be frustrating. The CDDB search function within EasyTag sure seems broken to me.

I tried something along the lines that I suspect you did and when I click on "Search" I get a message stating that "0 matching albums" were found. If I click on "Search Using Selected Files", I get the "404" error.

Suspecting that there may be more to using the GNU database than simply selecting it from the drop down menu in EasyTag's Preferences dialog. I changed the port number from "80" to "8880" (which is the port number I've used with the GNU database with cdda2wav(1)) and I get a different error message when clicking "Search" (same 404 error when clicking on the other Search option). If I revert back to gnudb.gnudb.org:80 I get the "404" error no matter which Search option I click.

The "How To" page at "gnudb.org" ought to help, right? However, when I tried using the proxy server mentioned on that page, EasyTag returned an error about not being able to resolve the server named on the GNUdb page. host(1) has no such problem, though.

I was, eventually, able to select an MP3 and do some sort of search on GNUdb.org by right-clicking on the selected file and selecting the CDDB Search option (NOT by following Miscellaneous->CDDB Search). I got a message that 153 bytes had been received... and not long after that, KDE announced that EasyTag had stopped responding.

I hate to say this but this would be about the time I'd be considering a switch to a different tagging utility. (And, come to think of it, the problem you're encountering may be part of the reason why I wound up rolling my own tagging function based on metaflac(1) for use when I was ripping the WAV files from my CDs.)

Surely, not what you want to hear but at least you know you're not the only one butting heads with (NotSo) EasyTag.

Thanks for the replies.
Am trying this in both Opensuse Leap 15.2 (fully patched - pacman), and Ubuntu 20.04 (fully patched).
Beleive both are running Easytag 2.4.3, and believe no updates for quite a while.
Realise that freedb no longer works, and have been trying gnudb with suggested options:
There is a post here that says it was working a while ago, and have queried there, but getting no helpful response.
Did try the : CDDBURL="http://freedb.dbpoweramp.com:80/~cddb/cddb.cgi" option, but this gives me an "Error resolving..."?

Would LOVE to be able to find an equivalent prog. to tag files, but unfortunately haven't found anything that comes close to the usability of Easytag...

That was an example from a different software. No, it probably wouldn't work if you used it exactly like that.
Are you sure Easytag can't be tickled to give you some useful troubleshooting output?
Can you reproduce the problem with other software? That would point to a problem outside of Easytag.