Mound System Solids Tank Backup

I have a two tank mound system. The first tank ins the solids tank. The second is the grey tank and ejector/distribution pump. In between is an inspection cover that is closest to the solids tank and about one foot away, towards the grey tank, is a steel pipe with a vent cap.

Yesterday on a chance inspection, I discovered that the solids tank was FULL, right up to the top of the riser.

The only explanation I have is that the transfer pipe between the tanks is clogged or even possibly frozen shut.
I am in the upper Midwest and we had a particularly long winter and recent super snow storm.

Last year I installed an anaerobic air pump system in both tanks.
I am wondering if pumping cold air 24/7 down there assisted in freezing the transfer pipe shut.

Today we have 50F+ and 60's on the way.

I ran a hose down the vent pipe and water comes out the grey tank.

I ran the hose down the inspection hole and I heard water on the solids side.

My guess is that the blockage is between the vent and the solids tank.

Just wondering what to do next...

Nasty business this is...

Well, I figured this one out on my own.

The two concrete vaults are placed right next to each other connected by about 2 feet of 4" PVC transfer pipe.
That pipe was indeed blocked.

I used two 4' sections of 1/2" PVC pipe connected with a coupler, a right angle fitting and a short 3" section of PVC to make a flushing tool.
I inserted the end with the right angle fitting down the vent pipe which is on the liquid or grey tank. I connected the other end to my garden hose.
Using a spotlight, I positioned the 3" section into the transfer pipe and hit the water.
It blew out a large blockage back into the solids tank. I could see that white waxy crap floating in the tank.
30 years of grease and soap had finally blocked that pipe.
Should be good for 30 more.....knock on wood.