Sorry forgot replying earlier, have been away for a while.
Setting full path and [0] in double quotes fails with with same error.
This is the only combination so far than I get problems with during testing with e.g. ' " \ •
The folders contains brand names and some of them are strange. I agree that the best would be to use only "a-z,A-Z,_" but I'm not in charge.
If I run the command with "%%%" (or "%%%%") instead of "%%"it works (seem like convert is doing %-replacement on paths as well). I guess this is the way I need to go.
Thank you all for trying to help me!