No local position preventing copter from going into Position Hold

Relevant log file review:

Why is there an error “Failsafe enabled: no local position”? What are the conditions when such an error might be produced? We are sending position data telemetry and using the EKF2_AID_MASK parameter to accept the data as vision position.

Also, I followed the instructions in and generated the pdf plots. In those, for some time the horizontal position innovation goes constant. Why is that? Here is the pdf: log_135_2019-5-30-18-31-28.ulg.pdf (127.3 KB)

Any help would be much appreciated!