Thank you, David. I found the following queries to check LiteSpeed license and version from your article "LiteSpeed Licensing Compliance: A How-To Guide".
EXEC master.dbo.xp_sqllitespeed_licenseinfo;
GO
EXEC master.dbo.xp_sqllitespeed_version;
GO
I ran the above queries against all three servers and found the problem on C server. I got an error message "Failed to login to SQL Server" from the second query on C server. So the credential does not work for LiteSpeed on C, but it works for database engine connection since I can open SSMS with that credential. I reported it to my network guy and hope he can figure it out.
Thank you again!