Well this is related because I tried to switch frontend language from french to english and now instead to get the error I mentioned I get this:
Please enter a valid username. No spaces, at least 2 characters and must not contain the following characters: < > \ " ' % ; ( ) &
I tried to set CB last_name field to "not required" and if I register only with first_ name without providing last_name, no problem and I can register in both french or english.
Furthermore I looked in the file methods.php and the code is the same in 2.5.14 and 2.5.16... So the problem do not come from this file.
So what to do? We must wait an improbable Joomla core patch/release and meanwhile user will be unable to register ? Is not possible to do something on CB side ? This is scandalous because J 2.5 is a LTS version and update are supposed fix know problem or security issue..but not to modify something which worked previously perfectly fine.
any clue?
The only positive thing is now I know I must NEVER update Joomla before first testing in a DEV site. This is what I do always when I update third party addon but I thinking we can trust in the LTS Core...