just upgraded to Current Version: 19.04.30 and ran into that issue.
My video files are often captured with variable frame rate so Shotcut offers to ‘convert to edit friendly’ versions.
In 19.04.30 I tried all 3 options and each conversion failed.
Videos I recognized it are taken with Android app ‘AZ Screen Recorder’ (free) where I let choose the app the bit rate automatically.
UPDATE: These videos do not have an audio track! According to Pauls post below that might be the problem.
In version 19.02.28 the conversion was not an issue.
I am using the portable version.