Error Pushing block - Red and yellow text

Hi All,

I am mining successfully on 2 machines...however I am getting error on a third machine, the error is about pushing the block, it comes up in red and yellow text. Please see sample pasted below. 

can anyone point out to me what the problem may be? 

Many thanks.

Sample of error:

468441ms th_a       fork_database.cpp:41          push_block           ] Pushing block to fork database that failed to link: 003c1351bf2c8             2b90a373d378e013e36892179d3, 3937105

468441ms th_a       fork_database.cpp:42          push_block           ] Head: 3936908, 003c128cf630b887705d26cc71b77237a5eb72a2

===========

68223ms th_a       fork_database.cpp:41          push_block           ] Pushing block to fork database that failed to link: 003c1292cda0c1             ca3adbe3d4cf418f7f126176ac, 3936914

68223ms th_a       fork_database.cpp:42          push_block           ] Head: 3936908, 003c128cf630b887705d26cc71b77237a5eb72a2

68224ms th_a       application.cpp:448           handle_block         ] Error when pushing block:

3080000 unlinkable_block_exception: unlinkable block

block does not link to known chain

    {}

    th_a  fork_database.cpp:62 _push_block