Could somebody tell me, what does this mean:
[swscaler @ 0x7f0980949240] Warning: data is not aligned! This can lead to a speedloss.
I have tried several times to export endpart of my movie to mp4 file and I get this statement and in jobs “failed”.
Ihave one videotrack and two audiotracks. I have managed to export in parts the whole movie exept this last part.
I solved the problem for myself. I opened the MLT as file and exported it after that to mp4-file. And everything is now ok. But I still do not understand, why the export failed.
To my knowledge, data alignment errors happen when variables in the code itself are stored in improper locations in the memory. There is a wikipedia page on it here.
It is said that misalignment slows down the application, or in severe cases causes it to stop working entirely. But I’m far from an expert in this area. My best guess is that portion of your project has been corrupted in some way. (I doubt this has any significance with the program or engine itself.)
Glad you could find a workaround though. 
In the case of Shotcut, this warning comes from an FFmpeg library telling us that something sub-optimal is occurring. It is not causing the export failures; the alignment problem is simply a lack of optimization at a very low level in the engine. If you want more detail, then it is simply because the rows of images in memory are not aligned on a 16 byte boundary. It can still work with it unaligned, but just complaining. It is difficult to address this now inside the MLT engine because the engine is shared by many plugins and apps that would break. We would have to add a new image type that is aligned, but then convert to & from something unaligned again for plugins and apps that do not accept that new image type. In reality, it is faster to just go the suboptimal route rather than do the conversions. Yes, that is getting deep into technical detail, but you asked. 
Sorry, but I do not know what caused your export failure. Apparently, whatever failed lacks the ability to log the error. 