As a first step, please could you try installing a totally fresh copy of your theme from the theme store. It will automatically install as unpublished / not live and it won’t affect your current live store. Make sure you are logged into your admin, then go to the themes store, find your theme and download. Then preview this unpublished theme to see if the error is still present. Please don’t use a duplicated version of your existing theme as this will simply contain the exact same code and prevents us from identifying the cause.
If the issue is not present on this fresh copy of your theme, the next step would be to isolate things further on your live store. Try uninstalling your apps one by one, starting with the most recently installed first. Then see if you can eliminate the issue this way.
Alternatively, if you are still able to replicate this issue in the fresh version of your theme, please let us know and also send through a preview link to the clean version of the theme. We will then take another look.
To share a preview link, please go to: Online store > Actions > Hover cursor over "Preview" or “View” > Right-click > Copy link