After upgrading a website from EPiServer CMS 8 to EPiServer CMS 9 we cannot upload files to the CMS in production. When trying to upload an image the status is set to Failed and when hovering over the status an error message “Object reference not set to an instance of an object.” appears. We cannot reproduce the error in test or development environment. When we restart the site in IIS in production, uploading files works fine for a couple of minutes and then starts failing again. I cannot find any useful information in the log file. I have checked the access rights to the folder on disk and it looks fine. I have also tried to debug the application against the production database but I cannot reproduce the error this way. Does anyone have an idea about what can cause this problem?
Is it giving the error for specific file extension or it is for all media types?
And please check the log files if it has something that is pointing to actual error.
Load balanced environment with multiple frontend servers?