I don't like

ComboBox1
ComboBox2
ComboBox3
ComboBox4

These look like boxes which do not have specific names and could be duplicated
on the other pages of the form. Also I would recommend using .text or .value
at the end of each of those lines so you know what you are getting. However
that may not be the problem.

I am not clear what RowNumber is. There are no DIM statements. Is it DIMed
as an object? Does that object have a text property?

How about putting a breakpoint on the line
If R > 1 And R <= LastRow Then
and then (when it breaks) floating the mouse over the R to see what value it
has. Or type ?R in the immediate window.

Then step through the rest of the lines (f8) and see where it dies.

Hey, Why is this a function not a sub? Try making it a sub unless you need it
to be a function. (Again, I don't know that this is a problem)