Site "crashing" after file upload (time and date is shown) on a white page

Our clients experience a problem that sometimes when they upload files, the page gets totally white blank and shows date and time after they click the upload button.

We experience this with different browsers (safari, chrome, edge) as well as different OS and devices. For some clients it works, while for some it does not.

we are desperate to find a solution as we cannot pinpoint to where this issue is coming from.

We only allow PDF and pictures. Filesize is not exceeded, so it can't be that.

did anyone have this problem?