I'm getting a lot of "An error occurred while changing your watchlist settings" error messages when trying to add pages to, or remove pages from, my watchlist.

On which exact website does this happen?
Anything displayed in the console of the web browser's Developer Tools when that happens?

Oops, sorry. This is on the English Wikipedia. It is still happening now, although not to the degree it was earlier today.

Am not having the same problem on the Spanish Wikiquote and English Wikiquote (arbitrarily chosen for comparison), but it is happening on the Hungarian Wikipedia (also an arbitrary choice).

Unfortunately, I have no idea what to look for in the Developer Tools on my browser, as I never use that, but I do see in the Console:

JQMIGRATE: Migrate is installed with logging active, version 3.0.1  load.php:139:615
This page is using the deprecated ResourceLoader module "jquery.byteLength".
Use "mediawiki.String" instead.  load.php:215:1070

before I even try to do anything with the watchlist. This was from the Hungarian Wikipedia, but I'm seeing essentially the same thing on the English Wikipedia. No warnings on the Spanish Wikiquote (only the first line about Migrate being installed), and the following on the English Wikiquote:

JQMIGRATE: Migrate is installed with logging active, version 3.0.1  load.php:139:615
Use of "wgAction" is deprecated. Use mw.config instead.  load.php:154:969
Use of "wgNamespaceNumber" is deprecated. Use mw.config instead.  load.php:154:969
Use of "wgCanonicalSpecialPageName" is deprecated. Use mw.config instead.  load.php:154:969

I'm not seeing anything on the Console that comes up specifically when I add/remove pages to/from watchlist, whether it succeeds or fails (but, like I said, I don't really know what I'm doing).

FWIW, I do not seem to have any problem removing pages from my huwiki watchlist using the Special:EditWatchlist interface (removed one page, then later removed two at the same time).

FYI, as I post this I'm still seeing the same problem on the English Wikipedia, although the failure rate is way down. Now when I get an error it seems like just waiting a few seconds and trying again will usually (but not always) be successful. Weird. I've posted to the technical Village Pump (which maybe I should have done before opening this task… oh, well) to see if anyone else is having this problem.

I wonder if some gadget / user script interferes... Does this also happen when using [[ https://www.mediawiki.org/wiki/Help:Locating_broken_scripts | ?safemode=1]]?

Thanks for letting me know about that option.

Well, continuing with the trend seen yesterday, just now I was able to "watch" a lot more pages in a row (about 8 or 9) before it started giving errors (this was before using your suggestion). Same thing when I "unwatched" all of them. So the failure rate is way down (again), but still not to zero.

Tried appending safemode=1 to the URLs and reloaded the articles. Added all of them to my watchlist with no failures.

Then I reloaded the normal versions of the pages (without safemode=1) and was able to remove all of them from my watchlist with no failures. Sigh...

So, even though that last experiment was somewhat inconclusive, it does look like there's prima facie evidence that the errors may be due to a script or gadget. (Initially I had thought that the fact that it worked sometimes was evidence that it was not due to a script or gadget!)

I'll investigate this further and let you know if I can figure out what's causing the problem…

(Whatever turns out to be the cause, the failure rate seems to be so low now that I may never encounter this again in the normal course of my wiki activities — I hardly ever need to add a ton of pages in a row to my watchlist.)

Well, I still don't know what the problem was, but I could not replicate it today, so I guess this task can be closed.

In case you're interested: Yesterday I disabled a few gadgets I didn't need, but I don't think I took the time to actually test watching stuff again (had to do other things). Today, unfortunately, I encountered a more serious problem where I couldn't even save any changes to a page I was editing (the "Publish changes" button was essentially working like the "Show preview" button — and no, it wasn't because I hadn't entered an edit summary), so I had to resort to clearing out my cookies and cache to fix that problem. (I hate doing that!) Just now I was able to watch and then unwatch a series of 13 articles with no errors. So, maybe the problem is fixed.