Why Does CorelDraw 2018 Sometimes Crash On Save?

I just recently upgraded from Version X4 (14) to Version 2018 (20). Almost all of my CorelDraw files are in X4 format (literally thousands of files). So far, they seem to open just fine in Version 2018 (20). However, when I try to save them in Version 2018, some of them crash the CorelDraw program, while others do not. Also, when I use the "Save As" feature, I am able to save those same (crashing) files in Version 14 (X4) and/or 15 (X5), but if I try to save in a higher Version, i.e. 16 or higher, the program crashes.

Is anyone aware of this problem and is there a solution?

Thanks for any help.