the reason is that the name of the output mpeg fie is not copied in the
"outfile" variable, which is used to write the name of the output mpeg in
the configuration file for "ppmtompeg"|"mpeg_encode" encoder. Therefore
the name of the output mpeg is blank and the encoding always fail with
"ERROR: Could not open output file!".
Tested with GRASS: 7.0.0, 7.4 svn (12/01/2018), 7.6.0, 7.6.1.
Works in 6.4.6 because the variable is not empty, see lines 442-445 of