Office 365 on Protected sheet written in Excel 2010

[SusaninHouston]

I'm running into an odd situation on a workbook originally written in Excel 2010. The workbook in question contains several sheets, all of which are password protected. A user running Office 365 for Business is using the workbook and running a macro that essentially does the following:

1 - Unlock several worksheets, each containing a combination of locked formula and unlocked input cells

2 - Change formulas in a range of cells (that includes both locked formula and unlocked input cells)

3 - Re-lock the worksheets

This VBA code was originally written several years ago and has worked reliably on several hundred PCs running multiple Excel versions. In fact, it also seems to work on this PC with this Excel version also, EXCEPT for this.

After running the macro, the formerly-unlocked input cells (ALL of them on the worksheet - NOT just the ones in the change area) now throw a protected-worksheet error as if the cells were locked by the macro.

And just to make it extra weird, clicking onto a different worksheet in the workbook, then back to the original unlocks the cells for input again...

Any ideas? I cannot control which version of Excel/Office my users use, but can ask them to install patches or updates.