I have been using the same excel spreadsheet for over a year, without any problems. For the past coupe of days, a column I have formatted as Number/Special/Zip Code, places an error (triangle in the upper left corner of most cells (but not all) in that
column only.
I go to the Excel Preferences/Error Ckecking and remove the checkmark in the 'Turn on background error checking' and the error indicator disappears until I shutdown and reboot (I can file the document and open it again, without problems).
This is on an iMac (Retina 5k 2017), running the macOS Mohave Version 10.14 with Office 365 with Microsoft Excel for Mac Version 16.17.
Any suggestions would be appreciated...