Fitness app not working

Same problem here. I’m using watch series 4 and iPhone XS Max. I have also noticed the heart rate monitor on the watch no longer detects my heart rate. The watch is able to detect a workout but can’t record any information about it. . The fitness app on the phone doesn’t track and calories, exercise minutes, or stand points, or any steps taken or flights of stairs climbed.

I had a similar problem but was able to fix it by resetting my watch (keeping my cellular plan) and then re-pairing the watch to my iPhone. The reset (unpairing) will wipe-out any accumulated awards, songs, data, etc. but, in my case, got all my Apps running again.

To Reset, open the Watch app on your iPhone. Click the General icon. Scroll to the bottom of the General menu to find the Reset feature and click it. Follow the prompts from there.

If your watch has a cellular plan, you will be asked if you want to keep your plan (from being erased). I recommend keeping your plan.

I hope this helps.