Ill bet it amounts to 'inappropriate remarks' and that not stating exactly what it is, instead using the 'sexual harassment' doublespeak is just a FUD tool to attempt to sabotage the Pirate Party.
Here's a link to what seems to be the party's response. From my reading, possibly tainted by translation, he agreed to step down if the European Parliament's Consultative Committee on Harassment at Work found that he had engaged in harassment, they did, and he resigned, but they were unable to remove him from the list due to regulations.
While I can't seem to find an account of what exactly transpired, it does appear that the European Parliament's Consultative Committee on Harassment at Work concluded that something did indeed transpire that falls under sexual harassment.
But even without details looking at Julia Redas track record and the fact that the German pirate party and European pirate party distanced themselves from him suggests more than a minor error in judgement. It’s unbelievable that he could successfully trick his party so he could get on the candidates list as number 2. And he will be there on election day! This is no FUD but something they have to blame on themselves.
(That blog entry is linked in this Buzzfeed article: https://www.buzzfeed.com/de/pascalemueller/europawahl-pirate... )