Timeout poll on interrupt endpoint for RPI3 with keyboard and mouse

[bob prohaska]

For the first time in a few months I connected a monitor, Dell keyboard and
mouse to my Pi3, presently running
12.0-ALPHA7 r338964 GENERIC arm64
which have all worked well since I got the Pi3 around six months ago.

With the peripherals connected, the serial console spews
Timeout poll on interrupt endpoint
and does not seem to accept normal inputs, so it isn't
possible to get into the debugger, boot an old kernel
or otherwise try to sort out what's going on.

disconnecting the monitor had no effect.
Removing the keyboard/mouse restores normal behavior. 

Anybody got a hint what might be wrong? FWIW, this OS/kernel
pair were compiled and installed with neither mouse/keyboard
nor monitor connected. I wouldn't expect that to matter, but...

Thanks for reading,

bob prohaska