We've added lots of caching with our generators to try and minimize the impact of Errai's code-generation on SDM refresh time. In this case, it looks like there is an issue with our cache logic for array marshallers. As a workaround if you find this problem happening even after restarting SDM, you should try deleting all the various GWT cache files. There will be some in gwt-unitCache, and also some in the folder used by the codeserver (the folder gets printed in the console when the codeserver launches).
I've filed an issue for this problem and will investigate it further.
Thanks Max, deleting the .errrai directory helped with this exception and immediately was also the solution for some other strange exceptions I've seen. The gwt-unitCache is cleared automatically anyway whenever I clean my project. Should "noincremental" and "noprecompile" also be enabled?