Encoder showing glitches in export

Hi there, 

I'm trying to encode a timeline of prores4444 footage to a h264 and up till yesterday this worked fine. Now, suddenly I'm getting stranger render mistakes on the sides of some frames in the video. No matter what export settings I use, this glitches pop up in 3 to 4 frames of the clip (see left side of the frame on the attached image). 

Does anyone have a idea why this is happening? Again, yesterday I had no trouble at all making an export.

Hope someone can help out.