I am experiencing the same issue after the new March update (I do have the same version as mentioned above).
Relationship view
In addition to net seeing the relationship lines, I can't even click on the different tables and move them around. It seems like there is some sort of grey box as an overlay as you can see in the picture above and below.
When I then switch to the "File" menu this grey box still stays there and I am not able to click on the options (unless I click on the area right next to the box on the left).
Relationship error
When creating a new layout, it is possible to move around the tables for a bit and the grey box isn't visible. After some moving though the box appears again. Realtionship lines also don't show when creating a new layout.
New Relationship Layout
This is an issue with the majority of my reports but not all of them. A few reports still work perfectly fine in regards to the relationship-diagram. The reports that still work all have less tables than the ones no longer working (might be a coincidence, I don't know).
UPDATE:
So I was actually able to "fix" this myself.
This seems like kind of a weird situation but in all of my affected reports I had this one realtionship that I had to delete, which made everything work again.
It was a relationship of one Table with only one column (entered that data manually, which I don't know if it is relevant) to another table whose column was a calculated DAX column. This was the only type of relationship that caused the issue.
Maybe this workaround "solution" helps others with the same issue.