Under certain circumstances, you cannot connect to administrative shares (e.g. C$) on remote computers, even though you use the right credentials. Accessing a normal (i.e. non-administrative) share works fine.

Access Denied to Admin Share

As described in MS KB article 951916, Microsoft introduced as part of UAC a feature called “UAC remote restrictions”. It filters the access token for connections made with local user accounts or Microsoft accounts. In other words, it removes the SID for “Administrators”. Connections made with domain accounts remain unchanged.

Solution:
add the following key to the registry and restart the computer.

Key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System
Value: LocalAccountTokenFilterPolicy
Data: 1 (to disable, 0 enables filtering)
Type: REG_DWORD (32-bit)

Add the entry using cmd:
reg add HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System /v LocalAccountTokenFilterPolicy /t REG_DWORD /d 1 /f