I deleted pages from my watchlist (mainly from the (Main) namespace) and changed the interval in special:Preferences to one day.
With this configuration I get result and the indication:
"1,348 pages on your watchlist, not separately counting talk pages. ..."
It is still *not* possible to display for example the changes for the (Main) namespace and for time interval 2 days (I manually changed the browsers url to days=2). I still get the heavy traffic error page with "503 Service Temporarily Unavailable"

hidemyself=1: using standard interface url https://www.wikidata.org/wiki/Special:Watchlist?days=3&hidemyself=1&namespace=&action=submit I c<n see three days backwords

Is this issue ongoing? Does it continue to happen if you turn on or off the "enhanced" watchlist (turn on "Group changes by page in recent changes and watchlist" and "Expand watchlist to show all changes, not just the most recent" in Preferences)?

Do you get a long code inside brackets ([]) (likely next to "Fatal exception of type"?

If so, please post that whole line, including the code.