Had a new error today:
Error sending [print file] response coercing to Unicode: need string or buffer. NoneType found.
After poking around I noticed that the GCODE file I had uploaded apparently got truncated, so that it ended with .g instead of .gcode. After renaming it, the print worked fine.
Doesn’t look like an appropriate behavior to me. I would expect Astroprint to notify me if it found it necessary to truncate a file name, and it certainly shouldn’t truncate the file type!