What if you made a brand-new MIDI. Then:
- COPY the first part of the original song into it.
- ADD the blank measures for your solo
- APPEND the remaining part of the original song
- DELETE the original song if you no longer need it

If there is an area on the disk dedicated to the original song, depending on the file architecture, I can see why the machine may not like the idea of "inserting" measures. (Because the expanded song may no longer fit in the same disk track and sector.)

I never mess with MIDIs. But from a computer storage perspective your machine may like this approach better. My $.02.