I'm also frustrated with Registrator Viewer (RV) and over the USA holiday weekend I spent time stepping through permutations trying to understand why RV is unwilling to fast-forward or save certain file groups from my X2 running 1.02.05 firmware.
I recorded an hour of parking mode video and was trying to view/save the front cam content at greater than normal speed (x4, x16, x64). There were many video files and many partial files (less than my two minute file size setting) because of motion detection. RV would load and group the files OK, but would only let me view them at normal x1 speed no matter what RV settings I tweaked (renderer, etc). Attempts to save the group into a single video file always failed with a "Different format" error. But these files were all from the front cam, so none of the video attributes should have been "different".
So then I stepped through trying to group smaller subsets of content. Sometimes view/save at greater than normal speed would work, sometimes not, depending on which files were grouped. Eventually I was able to isolate two files, both partials (less than the full two minutes) due to motion detection, that would both play at greater than normal speed as a single file, but would fail to play fast as a group of two files. And when grouped as two files, the save would fail with the "Different format" error.
Therefore I theorize that either RV is flaky or something strange is going on with the content of the video files as written by the X2. I know nothing about the internals of video encoding so I'm unable to troubleshoot or speculate in more detail.
If anybody would like to examine these two failing video files, let me know and I can make them available for download.