I had to make a chance to the rmf file format that is not backwards compatible. If there is someone who has lots of data saved for whom going through another format is a problem, let me know and we can work on a solution. Otherwise, be aware that updating IMP today will render your old files unreadable.
In detail, the problem is that I was using an hdf5 attribute to store the list of keys. Apparently there is a maximum size on how much data can be stored in attributes, and I was running in to that limit. As a result, the list of keys has been moved to a data set, which is something that can grow arbitrarily large.