Why was travel by boat between greenland and europe made increasingly difficult from about 1350–1850? geog 2050?

Because of increasing sea ice.

In climate change taking place since then, there has been an opposite problem.  Beginning in the late 19th century, a reduction of sea ice has been progressing in the Arctic regions.  The pace of ice mass loss has been accelerating especially since the 1970s.  In the first decade of the 21st century, Greenland's ice loss accelerated at a rate of 30 gigatonnes a year.