web site loads mobile homepage on desktop

Hi,
I just updated PHP to 7.4 and now I face a strange and very annoying issue.

Once I browse to my home page on a desktop browser, it loads the mobile version instead. But once I go to one of my sub pages, it loads the correct desktp version of that page. If I go back to my home page I get the mobile version again and vice versa.

On a mobile device it works as intended.

Any ideas on what′s wrong?

Any help is much appreciated!
Thanks!