It could be something in that particular MXD is corrupt. You could try two things.
The first is to run the "MXD Doctor" it normally comes with the installation of ArcMap. I am not sure if it comes with 10.6. Normally it is found in (using Window 7): Start -> Programs -> ArcGIS -> Desktop Tool -> MXD Doctor
I have had some success in using this tool to recover corrupted MXDs.
Another alternative method, would be to check the Data Sources of the MXD. It is possible that perhaps a layer in the MXD is creating the error. Open ArcCatalog and navigate to the MXD giving issues, Right Click and select "Set Data Sources". This opens a window with all of the data loaded in the MXD. Try checking each data source to ensure it is reading information from the correct locations.
I have had issues with some of my MXDs where I have used a base map. These MXDs often will not open and I either have to reset my PC or run through the steps above to try and avoid the crash on opening.
I will try that in Catalog.
– Zoran Jan 25 '18 at 11:00I ran the ArcGIS defragmenter, that did not help.
I tried the Catalog, right click for Data Source and was hoping I would be able to delete layers in there but I do not think that is possible.
– Zoran Jan 25 '18 at 14:32