I have experienced the exact same problem as mentioned in this post.
Excel thinking cells are locked when they aren't
I tried to ask for clarification by responding / reviving the thread but it appears this is not the correct protocol. I only came across this post after numerous search string searches and this is the only relevant "hit" I could find.
My spreadsheet is similarly complex and has been running flawlessly through Excel 98, 2003 and 2010. When I use Excel 2013 (and I believe same problem happens with Excel 2016) I find cells which are unprotected (for data entry) become locked following use of a macro button, typically navigating to view a graph and then returning. Properties of the cell confirms the cell is not protected.
Reading the above post and the Microsoft links, I have only a shallow understanding of the problem i.e. SDI / Single Document Interface.
The above referenced post received a solution that was acknowledged as having resolved the issue. Perhaps I did not fully understand but no matter where I placed the code snipped it did not help me.
The code offered was:
Private Sub UserForm_Layout()
Static fSetModal As Boolean
If fSetModal = False Then
fSetModal = True
Me.Hide
Me.Show 1
End If
End Sub
There was another suggestion to try: MyForm.Show False I did not know where to pace this similarly to the main suggested code to resolve.
VBA did not like me using "Me" (Me.Hide / Me.Show 1). I tried place the code snippet in several places ... the main opening macro, the navigation macro to and from the sheet that is affected, changing "Me" to the sheet name all to no avail. It is a sheet but since the unique variables are gathered here, it may loosely be viewed as a "Form".
I used to be a competent amateur but never an expert. I am appealing for a kind person to help direct me to overcome this "Excel 2013" problem. I have tried and failed many times and spent many hours trying to fix only to find the problem is because I'm using Excel 2013. There are even now no issues using Excel 2010 which I am able to access via a friend.
I'm sorry my first post is so detailed and hopefully not over-complex. I only wanted to give the correct information about the problem.