Workbook.open a filename with path and cell value in it

At time of opening this thread I thought it was just a problem with this line. But the questions made me think maybe it is something somewhere else in the script.

Only difference is between first code and the line in the complete code is:

Workbooks.Open FileName:="O:\Splitsen\" & Range("J2") & ".xlsx"

Workbooks.Open FileName:="O:\Splitsen\TEST.xlsx"