If you happen to be running DJVU Solo version 3.1 and you have some documents saved with DJVU Mozilla Firefox browser plugin you might end up with an error:
"djvu decoder cannot decode djvu files with version >=25"
The error is well known and described on djvu solo's website. Though the reason is explained and a work around in future files downloaded via the DJVU browser plugin is suggested. The page doesn't suggest a way on how to open already downloaded .djvu files failing to via DJVU Solo.
Thanksfully there is a fix, get away from DJVU Solo and start using STDUViewer –
Here is description of STDUViewer taken from their website:
STDU Viewer is a free viewer for multiple file formats. The goal of this software is to replace the multiple document viewers with a single one simple application. STDU Viewer supports TIFF, PDF, DjVu, XPS, JBIG2, WWF document formats. These are the most popular formats for scientific and technical documentation.
Additionally STDU Viewer supports FB2, TXT, Comic Book Archive (CBR or CBZ), TCR, PalmDoc(PDB), MOBI, AZW, EPub, DCX and image (BMP, PCX, JPEG, GIF, PNG, WMF, EMF, PSD) files.
Well enjoy opening all your DJVU files formats with superior STDUViewer.