Cannot remote into some servers

I use RDP Manager 2.7

I have Sonicwall at each location. I VPN using Sonic's client and then RDP locally into a server. Some serves work, some unable to establish connection. When i restart my computer some connections begin to work and then stop after a while. Sometimes when i establish connection and use my credentials RDP just shuts off. I tried to reinstall but nothing helps. It doesn't always happen but like today I can't remote into a few serves.

Any thoughts where should I look? I looked at Even Viewer on servers where it fails:

An account failed to log on.

Subject:
    Security ID:        NULL SID
    Account Name:        -
    Account Domain:        -
    Logon ID:        0x0

Logon Type:            3

Account For Which Logon Failed:
    Security ID:        NULL SID
    Account Name:        ADMINISTRATOR
    Account Domain:        

Failure Information:
    Failure Reason:        Unknown user name or bad password.
    Status:            0xC000006D
    Sub Status:        0xC000006A

Process Information:
    Caller Process ID:    0x0
    Caller Process Name:    -

Network Information:
    Workstation Name:    
    Source Network Address:    -
    Source Port:        -

Detailed Authentication Information:
    Logon Process:        NtLmSsp
    Authentication Package:    NTLM
    Transited Services:    -
    Package Name (NTLM only):    -
    Key Length:        0