SQL Server evaluation has expired (2008 R2)

I'm using SQL Server 2008 R2. I purchased the open license but installed evaluation copy.

Today, I can't start SQL Server just after rebooting the server.

Event Viewer reports "SQL Server Evaluation has expired"

I have phoned to VLSC twice and VLSC told me that I don't need to register to use SQL Server. There is no difference between using evaluation copy and authorized copy, in case open license has been purchased.

VLSC told me that it's a bug and direct me to ask this question to technical support instead.

How to solve the problem? Thanks.