It may be the MMU is more sensitive to temperature. I just ran about a 30 Min PLA run with the Thermocouple hanging from the middle of the upper horizontal bar. Bed temperature was 60°C. Max temperature was 30.5°C Printer is a Prusa i3 MK2S. I don't have a MMU feature to test with unfortunately.
See attached Pix of the test setup and readings.
Closing off the small internal port and removing any restrictor will maximize the airflow out of the enclosure but you may need to open a few holes in the enclosure to allow more make-up air to enter. ABS wants to have a high ambient temperature to keep from warping during build. It is also very sensitive to drafts. Increasing airflow into the enclosure would not be the best thing to that material.
Is the jam occurring in the hot end or in the MMU adapter?