Hi Jochen,
I ran in to this yesterday, we're working on fixing it right now. It's due to this:
And because Galaxy's converters use the `ucsc_tools` IUC conda metapackage, which still points to the old versions. I have an update here:
Once that is available you should be able to `conda update ucsc_tools` the affected environments. I am also working on switching all of the requirements so they depend on the individual ucsc-* packages from Bioconda instead of the metapackage so we shouldn't need to maintain it in the future.
--nate