Can you reproduce this behaviour in the webclient when you directly connect to https:// in your webbrowser and try to upload a file there or when using the mobile clients?
Was this problem introduced recently or has this always been around?
Also, need to say it, 5.31.6 is very old - you should definitely consider to update your insatllation to a newer version sometime soon (make sure to read the important upgrade notes since you’re using a rather old version, you will have to do that in two steps with first upgrading to 5.37.10 f.ex.)