Excel 365 Macro Validation Errors

Hello Fellow Excel Users, Experts,

I am new to this forum, hoping to get some help and contribute. For the moment, some help!

I have a macro enabled file (not authored by me) that I have used for a little over 6 months in Office 365 (for filing tax returns). From early Feb, however, something changed in Office 365 (my investigated guess) as both the newly downloaded and populated excel files of that format (and previously downloaded, populated, validated and completed files that obviously had macro-validated successfully in other months) started producing a "runtime error 13-type mismatch" upon running the validation macro, and further, upon saving the file and running further validation macro attempts, the validation macro starts producing spurious error messages that suggest that cells are not populated when in fact they are; errors that read something like "field Axx is mandatory", suggesting that you have left that field blank, when in fact I have populated it correctly...weird!. I was able to get a friend who has an Excel 2010 version (as opposed to 365) to run the validation macro successfully with no errors at all..... which suggests the problem is not the file but Office 365. I tried to get Microsoft Support to diagnose & help yesterday but they couldn't figure out what was the problem much less fix it-- so I was referred to this forum!

Does someone have a clue what could be the problem, and therefore how to solve this problem (other than permanently regressing to Office 2010)?

Much appreciated,

James