After loading the CCP it is transported to E000 in bank1. For this the bank is changed to io1 with 4k shared.
During this action the interrupt handler is enabled and there is a chance the interrupt fires while the wrong banking is in effect.
This trans$ccp should be surrounded with "di" and "ei".

(I ran into this problem while adding a U2 drive-image handler to the drive table, this causing a hangup in the trans$ccp.)

None of the CP/M implementations I've tried can update the (otherwise working) VDC screen on my C128, and it's hard to find any reliable information about how the Z80 accesses the VDC. Does the original CP/M access the screen through the 8502, like it apparently does with some other I/O functions? If so, does your version do it the same way? Or do both, as I suspect, access it directly through two memory-mapped registers at $d600/$d601?