Input output error in ClearCase diff. Improper arg

I have a user who is getting an error when they are trying to access another's view. When they try to do a diff they input/output error on xcompare in unix and in windows they receive failed to display version tree browser for element Encountered an improper argument on the windows side. I have no issues doing this in either of the user's view. They both are a member of the correct group and the view was created with that group. What else could be wrong?

It can depend on the exact version of your ClearCase.
For instance, this technote mentions (Windows)

When accessing a file or directory in the IBM Rational ClearCase GUIs, for example, ClearCase Explorer or the Version Tree Browser, that has permissions of 770, you will receive an error: "Encountered an improper argument" rather than "Access Denied" or "Permission Denied" if you are not a member of the group that owns the element.

While the users might be part of the correct group (ie their CLEARCASE_PRIMARY_GROUP) is the same as the Vob group), double-check their permissions and their view permission.
This technote details:

If a principal has not been granted access to a VOB, an attempt to access the VOB from IBM® Rational® ClearCase Explorer returns the error, "Encountered an improper argument."

This error might imply insufficient permissions.
To determine the effective ACL on the object, use the command:
cleartool describe -eacl vob:\vobtag \vobtag

Regarding the view:

cd /path/to/my/view 
cleartool lsview -l -full -pro -cview

A fix_prot on the view might be necessary: see an example of such a fix_prot in "While creating a view I got this error; cleartool: Error: Failed to record hostname in storage directory"