Blocking artifacts when scrubbing H264/mp4 rendered from Media Encoder

Hi, hoping someone can help.

All of a sudden, out of the blue, after working with the same source media and After Effects project for 4 months, my H264/mp4 test renders via  media encoder are glitching during playback.  

The rendered files begins to play fine in QuickTime player, but then soon become polluted with giant blocks of missing or corrupted data that accumulate in areas of motion.  

This is especially visible when scrubbing, which causes the blocks to multiply and stack up on top of moving subject matter until the image isn’t visible anymore.

The same files played back in VLC look much better, but still show subtle artifacts or off color blocking,

I’m using the same computer and Media Encoder render preset that I always have, which has never given me problems in the past.  The source media and .aep project are also the same, though I’ve been working and saving the project for months now.  Also, the master source clip plays back fine and looks great in After Effects and Final Cut Pro.  The problem only occurs after rendering via media encoder.  

Thanks in advance for any insights.