Am not sure where to post this, could also post this in the module forum instead, but here I go:

I came to understand that silverstripe 2.4 will say goodbey to prototype and favor jQuery instead. But I have noticed that the beta of 2.4 still uses prototype for i18n. Is this going to change?

If not, this brings me to the following point: If you use i18n you are using prototype and therefor you have to use jQuery noconflict if you also want to use jQuery.

I have noticed that at least one of the modules (the Newsletter module in its latest trunk - could also be the userdefined forms since you need to install that as well) assumes you are not using prototype, and it therefor uses $() instead of jQuery() to start it's magic in the document.ready function for the subscription page. I hope that people are aware of this and offer a way to circumvent this - or that we can stop using prototype all together at some point.

Upgraded 2.4 alpha to 2.4 beta , and now cannot access cms anymore (mydomain.com/admin).

It is stuck in the SS loading screen.

FF shows in the error console:

I see that LeftAndMain.js in 2.4 beta is totally different from 2.4 alpha. Could this be related to yurigoul's point?
(Btw i'm not using translatable for that site, but the newsletter module.)

CMSMain.js has 0 bytes because after one of the refactorizations all the code got removed, so this is more like an artifact. Got some projects running on 2.4, and they have this file empty allright.

Edit: had a look and maybe it is a problem after all, will check after lunch

I think it's fine, there must be another reason for your problems. I've just installed clean branches/2.4 and CMSMain.js *is* empty. It gets populated on 2.5 though. Maybe there are some modules that are causing trouble? Or maybe some custom cms modifications you have? (since the clean install performs fine...)