@dmertz , thank you for your explanation. With it, I was able to trace back to the K-1 data entry and correct the $18,199 value from 'contributions' to 'matching contributions'; that corrected the worksheet and cleared the error. This one is on me (although the imprecise nature of Code R on line 13 of the K-1 didn't help...co-mingling regular contributions and matching contributions is a bit clunky.)
But that brings me back to the fact that this is still a software error. While it is true that Line 6a of the Keogh/SEP/Simple worksheet may not exceed $22,500 that is NOT the error that my form was generating. I should have seen an error along the lines of "The Sum of Lines 6a and 6b may not exceed $22,500" along with an explanatory note much like what you provided, @dmertz : "If there is a value in Line 6b, you may want to check your K-1 data entry form to confirm if that value is a contribution or a matching contribution."
Thanks for the feedback...I was able to file this morning.
Note: as an additional frustration, the error was being generated on a Worksheet, not on a form I would normally submit. The changes necessary to clear the error and reflect the matching contribution did not change the adjustment value that landed on form 1040 or on form 1040 Schedule 1.
