Your comments

Yes, allowing users to sert their own password would be nice, I'll add it to my todo list.

User specific theme however are not something I'll do. Hiding the download link using CSS is only a trick that will not prevent anyone having basic knowledge of HTML to downloading the file. So building a "security" feature around it is not a good idea.
Hello,

there are no built-in statistics feature in Ubooquity, but you can quite easily count downloads using the grep tool on Ubooquity log file.

Example:

> grep -c "Served file" ubooquity.log
156

Don't worry, I often let a few days pass before answering messages too.

I'll keep on investigating.

You did not. :)
I was asking because there was a similar problem in earlier versions of Ubooquity with JPEG files. I had to use a specific decoding library to get rid of the pink overlay that appeared for JPEG files that were using a special kind of color profile. I just wanted to be sure that this problem had not reappeared.

As for Calibre, the problem seems different (no pink overlay) and in any case the platform used for its development is not Java but Python. It probably has something to do with color profiles, but I can't be of any help (I have no knowledge of Python).
It reminds me of this problem.

I don't know yet if I can find a solution as the problem might come from a linux environment issue (file names encoded in a different charset than the one your system uses), but I'll work on it and let you know if I find one.
I have been able to reproduce the bug with PNG having an alpha channel. Thanks for the very accurate description by the way, it helps a lot.
I'll try to find a way to fix it.

As far as I know the bug only exists for PNG files. If you have this problem with other formats, please let me know (Jim, your problem is with Ubooquity or with Calibre ?).
Bug reproduced on my side. I'll fix it before the next release.
Thanks for pointing it out !