This is caused by Strict Mode option (enabled by default).
When this option is enabled SPS will reject connections that use unrealistic parameters, for example:
- number of columns and rows of the terminal is equal or higher than 512.
- terminal window resolution is more than 8192 pixels in either direction.
SPS will also reject port-forwarding connections where the address of the port-forwarding request and the channel-opening request does not match.
Strict mode can also interfere with certain client or server applications.