I've been using dm-crypt most of the 15 years I use linux, never seen it just hang like that... Is this a spinner or SSD? I'm thinking physical errors perhaps - have you looked at SMART data on the disk to check its health? I'm thinking it's having a problem with a bad sector, but these tend to manifest different ways between spinners and ssd.
If SSD, make sure your firmware is updated, as these drives seem to be constantly updated for weirdness. Older SSD's would start getting wonky, then just die one day, usually due to wear. Use of trim is useless with encryption (and a security problem), so I always use Samsung SSD's that do automatic wear-leveling, and haven't lost a ssd since or have had any disk-based weirdness in general in a while.
-mb