GIO error for filenames with a colon
It appears that zathura is unable to open filenames containing a colon. To replicate the problem, download a pdf file, check that zathura can open it, then rename it to a string containing ":", and check that zathura will not open the renamed file.
At least one other pdf viewer (Evince) appears to have the same problem. The problem seems to come from the syntax for locations in the GIO tool, which uses a colon.