Headphone output level is generaly 1.4vrms.
Line Input level is generally .75vrms.
Obviously feeding it roughtly twice the intended voltage might cause problems...
It may be shutting down to prevent overdriving the ADC in the sound chip.
You should lower the output on the first computer to about 1/3.
The input on the second one should be about 1/2.
From there you can adjust up or down a bit to get equal volumes from both machines.
In general it is always safer to turn up the input rather than the output...
You might also check your speaker setup...
If you are configured for 4.0 through 7.1 the line in jack is often reconfigured to work as an output for the rear speakers.
(although this doesn't explain why it would work for a...