I shift failure warnings

Got this 2014 VNL 780, with D13, in August 2016 with 578000 miles on it. It’s been a good truck so far ,with 933000 miles now. Changed transmission fluid, and filter when I bought it.

Last four weeks I have gotten transmission system failure, and check transmission at next stop warnings on three occasions. Every time, it happened right after starting to drive, after a two-day break where truck had been parked for two days.

Two out of the three times it would not change from first gear. I pull over to look for codes but all warnings are gone within seconds before I can see any codes, and truck drives and shifts normally again. Third time warning only flashed for a second.

I have driven a 2200 mile weekly round-trip after every warning, and shifting is normal, trans fluid level, and temp are normal.First time this happened was over 10000 miles ago.

I have replaced transmission air tank pressure control valve as it had a tiny air leak and replaced the transmission speed sensor.

The weird part is I only get the warnings after having been parked for two days. Never had any warnings after 10-hour breaks or any other time driving.

Anybody experienced anything similar, or have any ideas?

Guess I need a trip to Bruckner’s, or could it just be some sensor acting up?