Specifically, this linked comment (Updating to experimental 2021-08-21-2314 causes many errors every move in recent experimental world/saves, but not world/save from 0.F stable · Issue #50977 · CleverRaven/Cataclysm-DDA · GitHub) from the issue shows which elements to delete.
Unfortunately, all the bad entries are going to be somewhere in the “queued_effect_on_conditions” line of your affected .sav file, which is just one super long line. (I edited the line before posting the version you see in the issue by separating the elements on different lines.)
Fortunately, they are likely all lumped together at the front, before the stuff you want to keep.
Be careful and make a backup before editing!