I have a guess:

In the history (right hand column) entry for your FASTQ file, click
on the pencil icon to edit the attributes. Try explicitly setting the
type of FASTQ, e.g. for Sanger FASTQ Galaxy uses the filetype
'fastqsanger'. This controls how the quality string is interpreted
(which is important for quality trimming).

You can also specify this explicitly when you upload a file.

For more about the different FASTQ quality encoding, see:
http://dx.doi.org/10.1093/nar/gkp1137
http://en.wikipedia.org/wiki/FASTQ_format

Peter