Dropbox is known to have issues with special characters. It's not like this is something new. I don't think that they care.
The problem with Dropbox
The user sent me a screenshot that a PDF couldn't be created for a subject. He thought that the problem were special characters like / \. Mail Archiver simply removes those 2 characters together with the : because those characters are used for paths on the different OSs. The user sent me the session logs and I immediately saw that he tried to save his PDFs to Dropbox. I tried to archive an emails with a subject he sent me and I got the same error:
Saving the email as PDF to the desktop worked fine. Saving the same email to Dropbox gave the error. What could cause the error?
Finding the root cause in Mail Archiver
An email is html - something you see in your browser. The email data is loaded into a browser component where it uses the browser to create a PDF. The PDF is opened and gets a header and a footer. The last step was the one that was failing. I made an example which worked fine. Then I saw something really odd in Dropbox:
I was incredulous. How could I make an encoding conflict with such a simple filename?
One of the nastier "tricks" with filenames is to have characters in different forms. It's possible to have an accented character either as one character "á" or as two characters "a´". I remember the first time I had such a problem because I thought I was going crazy. The 2 strings I wanted to compare were the same but the comparison was failing. Such fun!
However, the "magical holiday season email" didn't have any special characters. The email had an emoji which was removed by Mail Archiver.
Then I had the idea to check the paths of the PDFs in Mail Archiver and in my working example. I spotted the difference quickly: in Mail Archiver the name of the folder ended with a space. The example didn't have a space.
What Dropbox does is totally idiotic: Mail Archiver created a folder ending in a space. Dropbox removed the space. But Mail Archiver still had a reference to the folder with the space when it tried to create a PDF in the folder. Of course, that didn't work.
The bug has been fixed for version 6.1.1.