Corel Draw X5 .cgm export won't open

Received an 18 MB .cgm file with a request to try and reduce file size.  Opened file with Corel Draw X5 and discovered file had 32k+ objects - .cgm likely created from dwg/pdf export from CAD software.  Spent some time working with the original .dwg and new .cdr to reduce number of objects (performed lots of "welds").  Exported to .cgm, file size now down to 2.5 MB. This .cgm file opens with Corel just fine (albeit slowly).

Customer had complained about other files being in excess of 1 MB, so continued to try and reduce the file size further.  Eventually got a .cgm output from Corel that was 500 KB in size.  This .cgm won't open with Corel - get an "Unknown error occurred".  I tried all different combinations of .cgm export options, and all return same error when trying to open the file.

Any help or advice would be greatly appreciated!

I am using Corel Draw X5 v15.2.0.686, on Windows 7.