Is your CMS 1.7.7 as well?
It should be and if it is, please make sure you have fully qualified path to the CMS library in CMS settings. (library location section here Xibo CMS Post-Installation Setup Guide)
Other than that, the error - do you have any anti virus, other software that is accessing your player local library?
It seems something is accessing it and causing issues.
One other thing, you might want to check this rather lengthy topic - Upgrade to v 1.7.7 -> daily or sometimes hourly client freezes it might be similar problem to what you’re experiencing.