I had a similar issue on my CiviCRM fresh installation (CiviCRM 5.10.4) -> IIS10 webserver. Error used to happen on both Joomla (3.9.3) and WordPress (5.0.1) versions. The problem was that we see any relationship or activities table (DataTables warning: table id=DataTables_Table_0) So, what I did to fix this problem is to add the following values in the app web.config. Do not forget any of these or it won't work. <system.web> <httpRuntime maxQueryStringLength = "12288" maxUrlLength="12288" /> </system.web> <system.webServer> <security> <requestFiltering allowDoubleEscaping="false"> <requestLimits maxAllowedContentLength="3000000000" maxUrl="4000000" maxQueryString="4000000" /> </requestFiltering> </security> <httpErrors existingResponse="PassThrough"></httpErrors> </system.webServer> Hope this helps !