There's not a lot of details, but a post on Montgomery Gentry's website announces the cancellation of shows in Cleveland April 14 and Syracuse April 21.

We truly apologize for any inconvenience, but due to contractual issues that could not be resolved, Montgomery Gentry will not be appearing in Cleveland on April 14th or Syracuse on April 21st. Please contact the point of sale for a refund for your tickets for these shows.

The duo had been traveling with the PBR, Professional Bull Riders show.  The SRC Arena website still lists the show as happening.