Error page after clearing cache on Modificators tab

Hi guys,
I was given OpenCart (Version 3.0.2.0) site for support today. I needed to edit header in order to add GTM tag. I made the following modifications:
1) Modified 2 files:
* catalog/view/theme/journal3/template/common/header.twig
* catalog/view/theme/journal3/template/common/header.twig
2) Updated site but didn't detect any changes, so I went to admin/Modificators tab and press Clear button on top right
3) Refreshed the site and saw the error page attached (Journal Theme Installation Error. Make sure you have refreshed Opencart Modifications.)

I tried the following steps:
1) Pressed on Update button on Modificators page but it didn't help
2) Restored previous header pages => it also didn't help

Journal OCMod (XML) file seems to be missing. Check if it's present in the "system" folder. If it won't help, contact theme developer on the matter.