Trackster .len file error when displaying a dataset - genome mapping

I am using BWA to map a yeast WGS on a custom genome that I uploaded in my history.
When I try to visualize the mapping using Trackster and picking the custome genome build for the indexing, I get the following error:

“Couldn’t open /cvmfs/”

The reference genome is cleaned up with NormalizeFasta according to the guidelines, and I don’t know how to explain this error myself…any hints?