What text are you unable to see? Is the username/password page hard to use? Are you logged in, but unable to read the chat from other players?

Regardless, you should contact Support for assistance.

i have the same problem on battle.net can't see the password / login

It likely has to do with a glitch that occurs due to the background app which is responsible for displaying onscreen whenever your press the caps lock or num lock keys. What type of system are you running? It seems that each manufacturer has its own version of such software.

I had the same issue and was legit about to refund when I came across this solution in an old thread that only had a couple posts. Found and deleted the software and now it runs perfectly!

(too bad there is no one online xD)

That makes a weird sort of sense. Diablo (and Starcraft, and Warcraft 2 BNE) are well known to do weird things with their graphics output, especially around the battle.net UI. I could easily believe that having another process fighting for use of the display while the game wants exclusive access could cause rendering issues.

A related issue that affected some people was that flipping in/out of the game while on battle.net would sometimes result in the monitor drawing your desktop, but with little bits of the battle.net UI overlaid in the top left corner. Most of the UI was invisible, and it was pretty hard to read. Quitting off battle.net and reconnecting fixed that one, if I recall correctly.