Your comments
Probably before summer.
Just in case: your font files are actually woff files, not ttf files ?
Also, could you open your javascript console (F12 in yout browser) and check if there is any error when you change fonts to Open Sans ?
You followed the right steps.
You can post the custom CSS you wrote if you want me to check it.
Some epub files define the font in their own embedded CSS. In that case, fonts are not customizable.
Are you able to change the font of your book using the defaults fonts provided by Ubooquity ?
If not, this means the font is defined in the book CSS and cannot be changed.
I plan to try to replace the current HTTP server I use (NanoHTTPD) with Jetty. This would solve the header problem, among others.
If the file is not too big (let's say under 10MB), you can try email: tom at vaemendis.net
Otherwise, like you said, you can give me access to your server.
The easiest solution though would probably to upload the file to one a the many free file upload services on the internet.
If you can send me one of the comics that has the problem, I'll take a look at it.
The OutOfMemory error is a serious problem as it might affect almost any part of the application.
However I don't know if this is the cause of your problem.
To address it, you can try to allocate more memory to Ubooquity:
The warning is expected when Ubooquity discovers a new directory.
Customer support service by UserEcho
I found a bug related to fonts which happens only in Firefox, not in Chrome.
I don't know yet if it explains your problem, but there is definitely something to fix.