Yep, whoops error.
OK, bummer. Well, it was worth a try -- so, thanks for doing the legwork.
I still do think that the teammb object is where you need to focus on, but it's up to you which one you end up running tests on, as your original issue is going to be the same for both.
Here is your issue/s:
1) Your no-data errors: I believe this is happening because the syntax on your linking line is incorrect to the point where it just fails to link. i.e. The correct syntax is that "FIELD" shows up immediately after the equals sign, and when this doesn't happen, the line isn't recognized as a linking line, so in effect you just have a blob of text with no link. It gets ignored (you can get the same result by taking the "field:" part out completely, or deleting the linking line completely -- you just get no data.
2) Your Whoops error: You put the "FIELD" right after the equals sign, so it recognizes that it's supposed to link, but throws a whoops error instead..... whhhhy? Looking at the documentation, I'm finding this guide:
EXISTS:A:$$OBJCODE=<Object code of the Linking Object>
EXISTS:A:<Linking Field on the Linking Object>=FIELD:<Linking Field displayed on the Original Object>
EXISTS:A:<Target Object>:<Target Field>=<Your value for the Target Field>
So my guess is (no matter which object you aim an exists filter at), the field going into this spot needs to be on the original object, i.e. your task, rather than trying to call the project from the task.
Next steps? I don't really know what this task report does. You have at least 2 different ways you could go here.
1) can you start over and use a project report, filtering for the owner's team in this way, and pulling in task information via a collection in the view?
OR
2) is this reporting important enough to you that you would put a custom form on all tasks and use a field on that form to pull down the project owner's ID? I've definitely seen universal task forms, and it's reasonably easy to set up by configuring your templates and then forbidding any creation of a project without a template. But it does take that commitment and governance, so that's up to you.