Just a data point: I made an open jaw booking YYC–YUL–FRA outbound, RIX–FRA–YVR–YYC inbound on a Comfort fare. Tried to submit an eUpgrade request online for the outbound segments, but kept getting this error.
This journey is currently not available for eUpgrade. The booked fare may not be eligible or the request may be outside the eligible clearance window.
I called in to request the upgrade. Agent also encountered an error, but got it through after a short wait.