OK, I was reading few old posts about flask deployment, and I could see that other users had bugs when testing on the web version of Thunkable (here for example: [Solved] how can I fix a 403 error using the WEB API - #9 by muneer). So, I moved to test on my mobile (Android Samsung, but in the post I referenced they also report that it worked on Iphone), and surprisingly - it worked great when I tested on Thunkable mobile app. So, in conclusion, seems like there is a problem when testing/calling API requests as web app. For now, I will call it solved, although I would appreciate if you could help us find a solution to test it also on the web.
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