I too am getting this error. It shows in red on save and fades which doesn't allow me to capture the text, so had to take a screen shot.
"saving failed: ajax call failed: parsererror - syntaxerror: unexpected token F. try again while holding down the shift key"
I did import a large table, 2939 rows, 10 columns. It took a long time to save, initially. Took longer to save when I added a two column pre-sort
"aaSorting": [[ 3, 'desc' ], [2,'asc']]
then errored out when I changed to 3 column pre-sort
"aaSorting": [[ 0, 'desc' ],[1,'asc'], [2,'asc']]
hard to maneuver on admin page, scroll, until I collapsed the "Table Content" section.
I even had a WP fail during attempted saving ( I can't replicate so this is based on memory) but it said something about post size exceeded.
Will keep looking and keep you updated. Just donated a coffee to you before all this started. Gonna upgrade you to a beer here in a second