Does it help if you clear the cache by FTP rather than using the Manager?
I've experienced a similar issue and this is the only thing I did differently.
I didn't need to disable plugins but I know all the one I use are stable on 2.3.1.
No, unfortunately it doesn't help. To be clear, i can save, and it looks like it takes. If I leave page and come back, the changes are gonzo.
-N
Only other time I've seen that happen the cause was normal MODX usage triggering a mod_security rule at the data centre.
What do you see in the Net panel of the debugger in Firefox or Chrome? When I had these symptoms the POST request to save the changes returned 404. I doubt this is what's affecting you but with a look.