Probably some hop particles clogging the line. The most likely spot for this to happen is the poppet. Cutting the diptube shorter probably won't help things any. Disconnect the gas, vent the pressure, pull the liquid QD off, and remove the post to have a look at the poppet. You might also pull the diptube out to make sure it's not clogged too since the post will already be off. Remove any clogs and try again. If it clogs up right away again, you might consider carefully racking it to another keg with a paint strainer bag or nylon over the end of the racking cane to filter out the large debris.
For future reference, you really need to make sure most of the large debris is filtered out before kegging.
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