I realized this actually corresponds to a mechanism that had been implemented a while ago to co-register NYU recordings, but with different file names and formats. I guess the Polhemus digitizer was upgraded and the acquisition procedure too?
https://neuroimage.usc.edu/brainstorm/Tutorials/Yokogawa#Alternative
I updated the Yokagawa/KIT reader to support the "new" hsp/elp files:
https://github.com/brainstorm-tools/brainstorm3/commit/11e3b82c89a4e6e93dd77f3655335b922e15dc9a
Please update Brainstorm and try again.
Double-check carefully the registrations you obtain, as I may have left some bugs in there, I don't have anything to compare the results I get with.