HRO ETD (Clark Pump) locking up

[Alita]

Hello everyone.
I have been running my 12V HRO watermaker for about 6 years now. In order to reliably have water in remote places, I own two different clark pumps to swap around when one is acting up.
The old metal ETD seems more reliable, but the seals wear out quite fast.
The new plastic one - made by Sea Recovery - seems to have sturdier seals, but it has all kind of other issues.
Lately it is locking up sometimes when I start the watermaker. The water that is coming back from the membrane is just blocked dead, so my cat feed pump is building pressure way over designed limits.
Even worse: when it locks up I have to disassemble the clark pump and move the pistons inside manually to get it working again.
This happened twice now and I have no clue why it happens. When I disassemble the pump I find nothing wrong with it - other than both pistons being locked in the middle. I move one up and one down, restart it and it works... until it locks up again a few weeks later.
Anyone had a similar problem and/or knows how to fix this?