From a different forum:

"..it turned out to be an invalid item within the charitable contributions. Apparently i's have an issue. It did  on both of these. Retyping the organization worked."   

I assume they mean retype the name of the organization.  It apparently has to do with the letter "i" being used, in either the organization name and/or the property description.  So, I'd retype all entries on Screen 26.

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Also (tho I got the despised thumbs down for suggesting this earlier...) - IF the client is locked in 2017, unlock (after backing it up/copying it) reprocess, and try to proforma again.