Based on the example of Patrick, I can provide an answer.
The material of the environment
mdframed is saved in a simple
\vbox. Based on computed lengths related to the text height, the box will be split by
\vsplit. Your material can't be split, so you run into an endless loop because
mdframed repeats the splitting algorithm. Up to now there is no check for such situations.
In order to avoid the endless loop you can use the option
nobreak. However the result is a blank page and the output itself.
\vrule height 250mm