Some category pages are blank

Hello everyone, I have a problem I am trying to fix for days.

In our eshop https://divollo.gr/ there are some category pages and some admin pages that give me a white blank page?

For example this works: https://divollo.gr/product-category/%CE%B6%CE%B1%CE%BA%CE%AD%CF%84%CE%B5%CF%82-%CE%BC%CF%80%CE%BF%CE%BB%CE%B5%CF%81%CF%8C/
and this does not:
https://divollo.gr/product-category/%cf%87%ce%b5%ce%b9%cf%81%ce%bf%cf%80%ce%bf%ce%af%ce%b7%cf%84%ce%b1-%ce%b1%ce%be%ce%b5%cf%83%ce%bf%cf%85%ce%ac%cf%81/

Also some times I see the same in my admin panel. Things I tried:
– reach my hosting company, there is no memory issue
– debug the site, there are no errors showed
– change the theme, same problem exists
– checked my error log, only a PHP notice exists
– cleared cache etc
– resaved my permalinks in options

None of these worked. Any help guys? I am stack.

Thanks
Gio

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