I use DropBox to store client information and years ago I set up a custom field to link to that DropBox location via web URL. Now, whenever I go to edit the account or even just that field, it gives me an error and refuses to save the information. Here is a Sample URL that it kicks out ...
It says, "Please Enter a Valid DropBox." I also tried it using http: rather than https: and that didn't change anything. The issue either has to do with length of the URL string or the % character in the string.