another error is "the specified file contains no video data" although I can play it in VLC

Open VLC.
Media > Convert/Save.
Drag files onto input files box.
Convert/Save to whatever format you want.
Enjoy.

No. 22107
Well I gave that a shot but don't have subtitles on the finished product
I'm not sure what I'm doing wrong.

Also, tried to convert an MKV file that FormatFactory and AVS couldn't read (but anything else will play it) and just got an audio track and no video. Any ideas?

I normally convert MKVs to M4Vs so I can use them pretty much everywhere on every device, unlike MKVs. I've included subtitles too, though MP4 only allows a single subtitle track so if you want multiples you'll have to general a subtitle file somehow for the 2nd and later subs. I never cared that much though which is why I only know a single subtitle works.

check that the container really is mkv
sometimes people mislabel their file extensions (used to be very common on file sharing networks)
if you're using mpc-hc, go to file properties, then go to the mediainfo tab
the file container should be under format (below 'complete name' and 'above file size')
you can also use mediainfo

if the extension matches, sometimes there's corruption. versatile media players will get around those (and also ignore file extensions if necessary), but media encoders may not.
one way to try this is do a lossless stream copy. you could try it with ffmpeg, but if you're queasy about using the command line, use something like xmedia recode.
format: matroska video
then drag and drop your file. make sure you click on the filename at the top. the video and audio tabs should appear.
under video tab, for mode, pick Copy
under audio tab, for mode, pick Copy
idk if you have subtitles or not, but you can work it out
then click add to queue and click encode
it should rip out the audio and video streams and properly rebuild them into a new mkv file. this is pretty fast because it's just copying the streams without encoding anything. hopefully your output (which should be in your videos folder) will be readable in whatever next encoder you use

use handbrake

those crafty baguette people make a good converter