I had the same problem and it was related the the identity set to run the QBXMLREP2e object and the security assigned to the company data file. If the QBXMLREP2e process is set to run under the "Interactive" user account, and in my case it was because my app is a web app, it would run as the "IWAM_computername" account. Your network share does not recognize local machine accounts, therefore you cannot give permission to the "IWAM_computername" account to your company data file.
By changing the account which runs the QBXMLREP2e process to a domain account, you can assign the appropriate NTFS security permissions to the company data file on the network share so that this domain account can access the file.
