Seems to be a problem with lessphp, not been able to read sintax properly at some point in the v3.0.2 of bootstrap. (issue opened here)
If you keep using lessphp, you will need to downgrade from v3.0.2 to v3.0.0. Modify the require line for twitter bootstrap in your composer.json file:
"twitter/bootstrap": "v3.0.0"
Then remember to update composer after that