Encoding Nero Audio Files

My copy of Nero 7 was initially capable of converting wav into mp3 files before and after I bought the mp3 encoder add on. At some after this it now gives a message that it "Can’t encode file"and then lists the directory path and file name and, more often than not, freezes up the system.
I have installed several other trial encoders and all have rejected the Nero wav file. I have also tried to reconvert the original wav files that it was happy to convert earlier - without success.

Has anyone had the same problem? Or better still - know of the solution, please.

I received a response from Nero. They said it was a problem related to installing the ‘paid for’'copy of Nero 7 without completely removing the trial copy. Files produced during the period when both were on the machine still give problems with conversion from .wav to .mp3 but since clearing out the old demo copy - and the downloaded ‘paid for’ copy then reinstalling the '‘paid for’ copy all is well.
One point is that in deleting both copies I also had to delete the ‘mp3 add on’ that permits more than 30 mp3 recordings. Their emailed web site had expired but I still had the download NeMP3Enc.dll and just adding it to the Program Files/Nero/Core directory seemed to do the trick.