I am in agreement with MarcTB, this looks like an MTU issue. MPLS will not fragment packets upon ingress to an LSP. Your best bet to troubleshoot this is to just do a ping between nodes on either end of the MPLS network:
ping size 1472 do-not-fragment <dest-ip>
If you're unable to ping with 1472 (that's 1500 bytes minus the IP header), then your MTU is too low. If you control the MPLS transport network, raise your MTU on those transit interfaces to account for your IP MTU, plus VLAN header(s), plus MPLS labels. I normally set all my physical MTU in my core network to 9100 so there is almost no way I will run into an MTU problem. All the edge-facing ports are set to the default MTU.