Storing transients giving database error

Sorry if vague, trying to track down recent website slowdown reasons, and query monitor shows me this as a DATABASE ERROR consistently for two particular transients being added:

Following the stacktrace seems to be a dead end, why is this throwing an error?

The line that triggers it is from my custom plugin setting a standard html-filled transient (10,000-20,000 chars), which has worked fine:

set_transient( $transient_name, $html, $transient_seconds );

By the way, running SHOW FULL COLUMNS FROM wp_options works fine in phpmyadmin

I also checked wp_options and saw the timeouts for these particular transients stored, but not the transient/values itself.

Any ideas?

Thanks!