Merge table cells not working

It's been a few days now that the merge cell function doesn't in our Confluence instance. We have used merge cells before, and it worked fine, but now it seem impossible to use this feature.

Test Case:
1. Create a table
2. Select 2 cells
3. click Merge Table Cells

Expected Result: Cells are merged into one cell
Actual Result: Nothing happens. Cells are not merged.