Receiving "500 error" occurs due to some DBsync related field values are missing / incorrect at Users page layout of SalesForce.
Select the user as System Administrator in Salesforce under User page, then check the following fields and their values.
a. DBSync Id : <DBSync account username>
b. DBSync Passwd : <DBSync account password >
c. DBSync Server URL : < https://dbsync01.appmashups.com/qboev3 > or <https://app04.mydbsync.com/DBSync/>
d. DBSync Profile : < Project name of DBsync profile >
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