From the name of the parameter on MXP it was only regards the RTP (=udp)
xConfiguration RTP MTU ?
*h xConfiguration RTP MTU: <400..1400>
It might have been interesting to see the complete trace to see the message
which triggered the ICMP message. As its a feedback on a TCP packet
was it a h323 / sip message or somthing different? Did this message had a DF bit set?
On the C-Series systems it seems to set the whole system MTU (tc5.1):
xConfiguration Network 1 MTU ?
*? xConfiguration Network [1..1] MTU: <576..1500>
You could try to set it to 1400 and see what happens.
But maybe its something strange in addition in your setup, but like I said
with just the screenshot its hard to tell.