That is sometimes caused by incomplete registry data for the filetype being double-clicked. If it is that then it's usually a simple fix.
Please install Opus 12.2.6 (beta) and then use Settings > File Types, then in the window which appears use File > Diagnostic. Enter XML as the extension and it will create a zip file with the relevant registry data.
(The same diagnostic tool is also there in Opus 12.2.0, but may not work reliably after the Windows 10 Anniversary Update is installed. The newer betas fixed that after we discovered it was broken.)
Alternatively, if you'd rather not install a beta version of Opus, you can use the standalone FileTypeDiag to generate a similar diagnostic file in .txt format, with a bit less detail.
Please then email the file to leo@gpsoft.com.au and I'll see if I can tell what's wrong and what to do to fix it.