'Filter formula: "The requested operation is invalid"

I am starting with PA and I am following several tutorials and the docs. Currently I get this error in my OnStart event of the app:

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The requested operation is invalid. Server Response: LocalizedTexts failed: Expression "locale eq null" is not supported.

The message I get at runtime says that only "eq" or "ne" etc. are allowed. If I change the formula to use those, I also get errors even when I type. Preview works without issues, running the app finally throws this error. Please be patient with me, but I just don't get it here.

By the way I followed this tutorial for a multi-language app: https://youtu.be/t-yCb70FLwY?t=1391



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