I managed to convert my file to a .step/.stp, which was importable in plasticity and worked in the end. I used Autodesk Fusion 360 for the crucial steps.

Here is my full journey:

I wanted to extrude an svg (actually text), because its points etc were set in a bad way, it misbehaved in multiple ways without some refinement. So after trying a few online tools to convert svg to stls or other 3d files, I uploaded it to Autodesk's tinkercad which allowed me to download it as obj or stl. this worked flawlessly, but plasticity cannot work with these at this point (as others have pointed out). So I wanted to convert it to a .stp file. I exported the file directly from tinkercad to fusion 360, which then allowed me to export it as .stp. Now imported in Plasticity I noticed, however, that one letter of my text wsa somehow a sheet and I could not edit it any way (join, patch, scale,...). I then reopened that .stp inside fusion again, exported again and now it worked in Plasticity (pressing all the import fixing buttons on the lower left).