If you have a .groovy file on the classpath when running a groovy script, imports of the classpath located file from the main script (or other places) only work when the absolute path to the .groovy file being imported doesn't contain non-latin characters.
It doesn't seem to make any difference whether or not the file to be imported is in the default package (top level) or not.
This problem is easy to reproduce. Attached is zip file with a groovy script and a single dependency in a subdirectory called 'test+test'. If you unzip this and cd to the subdirectory and run:
groovy -cp . Script.groovy
... it will fail. If you cd up a level and change the name of the directory from 'test+test' to 'test' then cd back into test and re-run it, it will work.