White Screen when go to sign a document

Good Afternoon,

We ccurrently are having a few clients within out team who are struggling to sign.

When they click sign they end up with a white box that pops up with nothing else within it no x to close it down nothing! you have to close the whole program to regain access.

Have checked there CA's which are empty have checked they company details which are correctly filled in. Have tried the self helps on here.

We use adobe reader DC