There is a known problem if property mapping is activated and, in a single MS Office session, you create a new file with the Save As option, then check it out and check it back in. The check in will fail in this situation.
To avoid this problem, upgrade to Alfresco, or exclude read-only mandatory properties from the property mapping.
The problem is caused when some mandatory fields are not filled out, but are declared as read-only. This is typically caused by system properties (for example, Creator or Modifier) that come with some system aspects. You can avoid this by overwriting the includedAspectsPatterns configuration to include specific custom aspects only.