Error code H505 may occur due to several reasons in QuickBooks like a damaged or missing .nd file or incorrect hosting configuration settings etc. However, the QuickBooks error H505 can be easily resolved by trying a few fixes. · Firstly, you may try to create a new company file folder and copy .QBW file in multi-user mode in the new folder. Now you may open the company file and verify if the error code H505 resolves or not. · Another way to eliminate the problem is reconfiguring the firewall settings. For that, go to the Windows firewall settings and visit the advanced setting options . Right-click on outbound rules and click new rules . Click port and then click next . Now try to open QuickBooks in multi-user mode again. These steps should resolve the QuickBooks error code H505.