Error other than "not found" occurred during open(): OS ERROR MESSAGE
There was a problem opening a file. The "OS ERROR MESSAGE" is taken from the OS and tells what exactly caused the issue.
A common cause is "Permission Denied" which means the rsyslog process had insufficient permissions to open the file. This can be caused by several things, with the most common being
- access permissions to restrictive
- SELinux policies (or similar systems, like AppArmor)
Note that in the case of SELinux and friends running as root does not solve the permission issue, because it is a design goal of these tools to be able to restrict permissions even for privileged processes.