I’d say it looks like you’ve got something mis-configured on the server, or elsewhere in the network. For whatever reason the MX is seeing packets with the 192.168.50.90 IP address on its interface for VLAN 1 - so there is most likely nothing wrong with the MX, it’s letting you know about an error elsewhere in the network.
What OS is the server running? Are any of the network interfaces bridged together, are there any ‘clever’ network functions configured in the network or on the server (e.g. NAT)? Are the VLANs bridged together somewhere in the network?