duplicated the problem today by adding another month of old data and adjusting the Opening Balance Equity.

first i added the adjustment, which made taax paying possible.

then by deleting all sales tax records and the journal entry for the adjustment the negative went away.

my guess is that adjusting the Opening Balance Equity and adding new sales receipts triggers a recomputtion of sales tax paid causing an over payment internal to the system.  at least i have a reliable workaround to clear it out completely now.