I recently experienced this issue, too. It seems to happen with large enough histories when Galaxy populates the input selector drop-down list with the first few items, then tries to load more and kind of gets stalled there. Don’t know if this is Galaxy Europe-specific or if you can reproduce this on the U.S. server.
@12Benne as a workaround for you, in case you haven’t discovered this yourself, you can always just drag and drop the dataset you want to use into that field of the tool interface.
We need to fix the underlying issue though.