Solution found: see this thread.

Thanks for your feedback. It will allow me to update the FAQ and the installation instructions for Synology devices so that other users don"t have the same problem.

Ok, installation done. I have been able to reproduce the problem.
And then fix it by adding the following Java parameter:

-Dfile.encoding=UTF-8

Could you try launching Ubooquity with it and let me know if it solves the problem for you ?
Your command line would be something like that:

java -Dfile.encoding=UTF-8 -jar Ubooquity.jar -webadmin

(you have to be in the same directory as the "Ubooquity.jar" file)

The connection probably fails because Ubooquity is not running anymore (that's what your log says).
As to why it stops, I'll let Synology experts answer as I don't own one myself.

Thanks. :)

For now I'm trying to install DSM on a virtual machine. I'll see how it goes.

I guess so (haven't tested yet).

Could you try to run Ubooquity in another directory (just for a test), scan a few files and check if you still have the same problem ?

Sorry, no idea. :(

There is still a modification I haven't tried yet, which (I hope) would allow files with umlauts to be displayed (although the umlaut would probably appear as another character or a question mark).
I'll try to include it in the next release.
I guess it would still be better than the current behavior.