Enable encryption (no reaction from the UI or in the log)
Another attempt:
enable old Master password on Linux (OK)
Enable encryption (no reaction from the UI or in the log)
During this time I had the Windows client off. Later I enabled it and synced from S3, and there I get a warning about Master password needing attention (although it also says it's valid, and encryption is Disabled, so I don't know what might the purpose of that reminder).
I guess this is better than 2.9.17 which just wouldn't stop spinning my CPU no matter how long encryption was on.
I read that one as I was struggling with constant decrypt batching in 2.9, and because of the "solution" from that post I uninstalled my Joplin and upgraded.
But I'm still seeing this the problem he had in my 2.10 and I thought it's supposed to be solved in 2.10. Anyway, I'll watch Github for any changes or fixes.
I would not mark my post above as a "solution". I just added a link to another report about this matter so that someone who can solve this is aware that the issue has been replicated by other users.
Same happens if the master password is set before trying to start the encryption wizard.
EDIT:
Console log on button push (twice)
C:\Users\vbox\AppDat…r\gui\dialogs.js:16 Uncaught (in promise)
TypeError: smalltalk.alert is not a function
at Dialogs.<anonymous> (C:\Users\vbox\AppDat…ui\dialogs.js:16:29)
at Generator.next (<anonymous>)
at C:\Users\vbox\AppDat…gui\dialogs.js:8:71
at new Promise (<anonymous>)
at __awaiter (C:\Users\vbox\AppDat…gui\dialogs.js:4:12)
at Dialogs.alert (C:\Users\vbox\AppDat…ui\dialogs.js:15:16)
at C:\Users\vbox\AppDat…figScreen.js:181:37
at Generator.throw (<anonymous>)
at rejected (C:\Users\vbox\AppDat…onfigScreen.js:6:65)
C:\Users\vbox\AppDat…n\lib\Logger.js:190 19:11:45: e2ee/utils: toggleAndSetupEncryption: enabled: true Master key null
C:\Users\vbox\AppDat…r\gui\dialogs.js:16 Uncaught (in promise)
TypeError: smalltalk.alert is not a function
at Dialogs.<anonymous> (C:\Users\vbox\AppDat…ui\dialogs.js:16:29)
at Generator.next (<anonymous>)
at C:\Users\vbox\AppDat…gui\dialogs.js:8:71
at new Promise (<anonymous>)
at __awaiter (C:\Users\vbox\AppDat…gui\dialogs.js:4:12)
at Dialogs.alert (C:\Users\vbox\AppDat…ui\dialogs.js:15:16)
at C:\Users\vbox\AppDat…figScreen.js:181:37
at Generator.throw (<anonymous>)
at rejected (C:\Users\vbox\AppDat…onfigScreen.js:6:65)

In v2.10.6 (pre-release) this looks fixed i.e. clicking on "Enable encryption" does trigger encryption actions.
How to encrypt, setup of Master keys, etc. is out of scope here, so for me this issue has been solved.
(Also, v2.10.6 looks better-behaved in terms of batch encrypting data than v2.9, so it's possible there have been other improvements related to encryption.)