Looks like Google finally did it. They don't say as much, but for a while google docs have all been popping up "this is no longer a supported version of chrome, upgrade now". But as of today, loading even an empty sheet in google sheets in chrome v68 results in "server error, please hit reload". Even on fresh installs or other systems or incognito. Happens right at the end, as the controls all finish loading. Planned obsolescence SUCKS.