This problem occurs when connecting an ISO external keyboard to a Macbook as an additional input. Some keys (the key under Esc and the one on the right of Shift) are incorrectly mapped, so they send unexpected symbols on the remote desktop. Such issue doesn't occur on all keyboard models and it is caused by a wrong keyboard layout type reported by the system.

The solution consists in editing the client configuration file and changing the following value:

<option key="Ignore the keyboard physical layout" value="true" />