After digging for a while, I found that the error was coming from the mixin.less file where the #grid stuff was getting defined.
This is what the file had originally:
(~".span@{index}") { .span(@index); }
After reading the LESS change log, I found that they changed the syntax so you can now use variables directly without needing the ~ hack. So I changed my mixin.less to look like this:
.span@{index} { .span(@index); }
There are a couple of other lines that you need to change, but they all follow the same format.
(~".offset@{index}") { .offset(@index); } changes to → .offset@{index} { .offset(@index); }