Hi Matteo,
for the second error, this would occur if no files were created during the spike-sorting process;
this means that either you have low sampling rate on your raw data, or there were absolutely no spikes on ALL your electrodes (highly unlikely). Try to lower the threshold on the parameters option on the main UltraMegaSort2000 window: spikes.params.thresh
The first error seems to be inside the WaveClus spike-sorter and I've never came across this:
can you send a link to download one example .edf file that you used to take a look?
Thanks,
Konstantinos