1. First, determine if the problem is really that you cannot connect or you cannot logon. The difference is that if you cannot logon you get a login error that username or password is wrong - if you get that please look at this post 
2. if you get a connection error please check if the VisualCron Service is running - if not start it and wait 2 minutes before reconnecting. In order to determine if the service is running, please open Windows Control Panel -> Administrative Tools -> Services. In the list of services, locate VisualCron Service.
3. if you still cannot connect then open the server_startup.txt located in the log folder of installation folder. Look for any errors:

Error:
A: It is likely that certificate 'CN=vc' may not have a private key that is capable of key exchange or the process may not have access rights for the private key. Please see inner exception for detail. ---> System.Security.Cryptography.CryptographicException: Keyset does not exist

or

Error at clientthread: System.ComponentModel.Win32Exception: The credentials supplied to the package were not recognized

There is a problem with a certificate that VisualCron is using for communication - please follow these steps:
A.1. download CertCleaner.zip  and extract it
A.2. stop the VisualCron service
A.3. run the CertCleaner.exe file
A.4. start the VisualCron service

4. if you do not see any errors then please send the log file to support through Contact  page

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