I just ran into this problem too. I was creating a data shortcut for a surface when C3D froze and crashed. When I went to reopen C3D, the program would not initiate and always got hung up in the same place you mentioned......loading AECC BASE UI. I restarted the computer with the same result. Knowing that the creation of the data shortcut caused it to flop in the first place, I deleted the xml shortcut file. When I did this, C3D initialized straight away.
This is a legacy project that I inherited with a bazillion folders/directories. The CAD data is being saved 15 layers deep in the directory structure at this point. Then you ad the ridiculously long xml file name C3D creates on top of that and I wonder if things are getting hung up when initializing as it trys to see the data shortctus and/or working folder when burried deep in a directory? Remember when you open a drawing related to Project A, B, C, or Z that the data shortcuts area in the toolspace is always looking at the last working folder/data shortcuts you told it to. Project DWG files are not automatically linked to a working folder/specific data shortcuts as the SDSK project did in Land Desktop.