Got an strange text in header, don’t know how to remove it

Hello. Some days ago, I joined this website mentioned in the topic and I noticed that nothing is working as plugins (Jetpack, Rank Math, etc.) and as you can see, there is an small text ‘is’ (header, up left).

I changed the theme to another one and everything worked (normal header and plugins work good). Do you know what is the problem? Thanks in advance.

The page I need help with: [log in to see the link]