I forgot to tell... After the verifying error in Nero I updated my w4824A to 1.04 firmware. Nothing changes, only Verbatim CDRs are verified at the full speed. I never had another verifying error, but also didn't burn another CDR of that brand. Maybe Plextor cd burner can't verify the disc at 48x, but why does it slow down to 4x or, in the worst case, can't it access the file?
By the way thanks for your suggestions.