MMC command via Behringer U-Control UMA25S not working and warning

i’m using Ardour with a Behringer U-Control UMA25S.
i would use the MMC commands to control transport, but ardour behaves in a strange way!
some keys work, and with some keys i get a warining:

[WARNING]: Global varispeed cannot be supported while Ardour is connected to JACK transport control

what does it means?
why MMC keys are not working good completely? are standard MIDI messages…

thank you!

It means that the controller(s) that are mapped to the “Shuttle” in ardour will not work if ardour is synced via JACK transport. Select “Internal” in the Transport and you should not get that message anyomore. The shuttle is the long horizontal bar at the top left of the editor.

To slightly expand on Joe’s comment: when Ardour is synced to JACK, there are only two possible transport states: stopped, or moving forward at normal speed. Anything that seeks to tell ardour to move forward at anything other than normal speed, or move backwards, is impossible to obey when Ardour is synced to JACK. Since there are MMC commands that tell the target to fast forward or rewind, that’s one possible reason for the message.