My PS is 1.7.6.8 same error. Spent weeks trying to find an answer.
Update your PHP to 7.4.x it fixed the problem on 3 of my stores.
But the backend formatting gets messed up on product edit/add, customer edit and a few other pages. (CSS issue)
Workaround, I put the store on Maintenance change the PHP to 7.3.x make my edits, put PHP to 7.4.x and then re-enable the store.
Update your modules too.
Hope it helps.