Interesting. There is no sump filter in the AW55-50. There is a filter behind the valve body, but that is a MAJOR pain to replace and will not solve your problem.

I suggest flushing the ATF (3309) using the engine driven pump (not an external machine with high pressure). If the flush doesn't help and you still get the error code, a solenoid cleaning/replacment will be easier than getting a tranny rebuild.

Send the filter and gaskets back, you really don't want to replace them.
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My back feels better when I sit in a Volvo seat

Here's one of several videos to flush your tranny fluid.

If it's brown and burned you want to flush it, and make sure it's red.
I used the Walmart full synthetic in two 850's. where there's a different clip
for that tranny plumbing, just get the right clip and o-rings at your dealer.
I didn't drop the pan at all in the 850's.

https://www.youtube.com/watch?v=leBtEnEDAtc

Some people have had success with solenoid replacements, which is preferred over
tranny replacement. Use the search engines to find the tranny solenoid posts
and videos. I have good luck with www.startpage.com

Bill