We have solution for this issue.Try to do the following steps.
Step 1: On the IIS Server , select the Application Pools in left side.It will list all application pools in right side.
Step 2: Do right click on Solarwinds Orion Application Pool select Advanced Settings , there is a property called "Managed Pipeline Mode".
Step 3 : Change the Managed pipeline mode property from Integrated to Classic
Step 4 : Give "OK" button.
Step 5 : Then Restart the service(stop and start)
Issue will be resolved by settings this property. Issue raises even after following the above steps then you may attach the Excel which is used for import.
for your reference. please refer attached screenshot.