Unfortunately, this error message is quite generic. Its meaning is that the JavaScript that runs in the block editor (which expects to receive the server response in JSON) got something it wasn’t able to understand.
In most cases this means that the server for some reason sent an error message.
If you in your browser open the developer tools and watch the “console” then you may see more information about what was actually received.
But I’ll make a quick guess first: Check that you have updated both the WordPress address and the Website address under Settings → General?
Also: How did you search and replace URL’s? Some settings are saved as “serialized data”, which is a special format, declaring “length, name, value”. If the length of your saved URL changed here, but the “length” declaration wasn’t updated, then you’ll have broken settings in many places. Easiest solution in that case may be to go back to the migration file and try some serializing-aware solution for search/replace (there are WordPress plugins for this, for instance).
Thread Starter
steehl
(@steehl)
Thanks for the first hint.
the error code generating the JSON message ist an http 500 error.
(service temporarily not available)
An interesting point this morning was, that I could do approx. 20 edits with no issues and then suddenly the problem reoccured. After that I couldn’t even redo edits which were successfully saved 5 minutes before.
Moderator
Yui
(@fierevere)
永子
service temporarily not available – is 503, not 500,
503 is caused by hitting resource limits on server (like connection limit or overall account limit)
500 is caused by script error, or memory allocation error,
error 500 should be logged, please check your PHP error log on server
for anything with “Fatal” at the end of log file.
Thread Starter
steehl
(@steehl)
We finally found the problem: WordPress is running on a hosted service at STRATO. Strato also has a guest-book spam filter included in its service. This spam-filter blocks suspicious post requests by returning a 503. For some reason it sometimes (not always) detects wordpress editor save request including links in the content as such suspicious post requests and blocks them.
Deactivating the spam filter solved the problem.