mp3 to mp3 resampling fails

trying to resample mp3 results in not valid multimedia files

windows media player prompt a 'no valid multimedia input file format'

vlc crashes

I tryed with different encoder options:

channels: join, stereo, force

cbr (129, 192)

vbr

include crc (yes/no)

...

always same result: the format of multimedia input file is not valid

analizing output file with GSpot the only quite strange field I can see is User Data / Metadata

it always contains a (maybe random?) string with no meaning

ex: [uSER] bJIM1Z

neither windows media player nor vlc can render the file

but creative mediasource can...

please feel free to ask for more details if needed to solve this issue

thanks for your kind cooperation