In IIS could you verify if a couple of files are listed. Below is the screenshot for them. They are normally what causes that error if they are missing or not properly configured.
Also, since this is quicker, could you verify if ASP.Net is registered. To do so:
Launch Command Prompt as Admin
Go to this path:
C:WindowsMicrosoft.netFramework[64]V4.xxaspnet_regiis –I
*Typically the version is V4.0.30319
Hit enter and it should register ASP.Net
To verify files in IIS:
Open IIS
Extend Sites
Extend Default Web Site
Single Click on DataServices
Double Click on Handler Mappings
Verify below files are listed exactly as they show here:
Afterwards try to launch dataservices/clientservices/securityuser.svc again and verify if you still get an error or if it shows properly. If you have any questions regarding any of the listed steps please let us know.
Thank You,