Sharing of Health Data with the fitness app "MATS" does not work

Hi everyone,

I recently installed a fitness app called "MATS" which amongst other functions analyzes workout data. It connects to Apple Health and Apple Health is feeding the data to MATS. While the data connection initially worked but was lacking historical data, I attempted fixing this by removing the Apple Health sharing connection in MATS and reinstall the app.

After reinstalling however, the connection with Apple Health is no longer established. When activating in the MATS app, it states that an error has occurred when establishing a connection with Apple Health.

What I find noticeable is that when I look into the sharing-section of Apple Health, the app "MATS" is no longer shown there. It used to be when I initially installed it.

Tried re-booting, re-installing MATS. Would anybody have an idea how this can be fixed? Thanks!