Dear mrgreaper,

The issue is, that i have an optional link to Belts+, thats causing the error message. The presence of Belts+ is not necessary for this.
Of course, if one wants to continue combining Belts+ with FuPower, both the link as well as the mod have to be renamed.

Do you have 0.0.12 ...

@Duros1394: Yes, confirmed, the cause is the "+" in the Belts+ mod's name. This is new and the solution so far is s simple rename of the mod.
Since Belts+ hasn't seen an update in ages, I offer here an inofficial version 0.0.3. Only difference to the official 0.0.2: the name ...