Excel file corrupt

I have been using GF EiE on a number of sites and love it. But on me recent project it only exports corrupt Excel files. When opening those, Excel asks if it should try to restore the content and on my machine that works. On our clients machine it doesn’t, so I kinda have to fix this. I have been looking at past answers from Doeke and checked my function.php and other files for errors like the closing php argument, but there’s nothing there. This is on an up2ate install of WP 5.6.2, GF 2.4.22.1, GFEiE 1.8.8. and PHP 7.4.15.
I am now cloning the site in order to deactivate all other plugins and use a standard theme, but if anyone else has some other suggestions, I’m all ears ??

Cheers,
Michael