Found it!!!
After longs months of pain and suffering (well, maybe it wasn't that much, but it caused a lot of headaches) I found the solution for this.
As well as wrsow I was getting one of those nasty: Sys.Webforms.PageRequestManagerParserErrorException
You're gonna owe me one Locopon =)
Sorry for the "harsh" entrance, but I just left like Arquimedes on the bathtub.
The problem with the localizator and the update panels comes from the Replacement you do when you install the hack in the Default.aspx.vb inheritage.
It's quite easy that you haven't found the error yourself while testing because you need to have a very particular combinations of settings for this error to show up:
- First of all you need AJAX Enabled in the Host settings (no Ajax, no errors)
- Secondly, you need to install the hack, use Tab localization and other general settings enabled.
The main problem comes with you combine the Localized Replacements with the URL replacement and the Solpart menu replacement.
I don't know how many of those things is needed to cause the error, but I'm avoiding it right now with: Hack Installed, Tab Localization Enabled, Solpart menu Replacement Enabled, URL Replacement DISABLED (I think this one is the key, but I'm not 100% sure), and I do use Localized replacements and also got the Ajax enabled.
Hope this works out for everyone.
PS: I posted this is English so wrsow could understand it, but if there's something you don't understand from my English yourself, I'll post it in Spanish.