MAVSDK: fly_qgc_mission issue: Mission int messages not supported

When trying to upload a QGC plan file via fly_qgc_mission I get the below error:

[07:50:24|Info ] MAVSDK version: 0.21.4 (mavsdk_impl.cpp:25)
Connection URL: udp://:14550
Importing mission from mission plan: testmission01.plan
Waiting to discover system...
[07:50:25|Info ] New system on: 192.168.42.1:14551 (udp_connection.cpp:263)
[07:50:25|Debug] New: System ID: 1 Comp ID: 100 (mavsdk_impl.cpp:390)
[07:50:25|Debug] Component Camera 1 (100) added. (system_impl.cpp:401)
[07:50:25|Debug] Discovered 1 component(s) (UUID: 1) (system_impl.cpp:561)
Discovered system with UUID: 1
System ready
[07:50:25|Warn ] UNSUPPORTED mission item command (20) (mission_impl.cpp:1541)
Found 2 mission items in the given QGC plan.
Uploading mission...
[07:50:25|Warn ] Mission int messages not supported (mission_impl.cpp:366)
Mission upload failed: Error

What does “Mission int messages not supported” mean? (My mission is a trvial mission to fly to one waypoint created in QGC).

Is the “Mission upload failed: Error” due to the above warning or might be unrelated (as it is a warning and not an error)?