Sorry to hear you're running into an issue with SA!
I'm very glad you've found a workaround by downgrading, but I don't immediately know why this shouldn't work with the latest version 6.8--hopefully we can find an alternative workaround to downgrading. To help us better understand first what was going wrong, can I please check:
-If you turn off all options and just process your assembly through SmartAssembly, does the error still occur then?
-Should the simple SA-processed version still work, could you kindly try turning on merging/embedding, obfuscation, and strings encoding features one-by-one to see which particular one is causing the error?
-Would you be able to share your application with us or a cut-down version? That would be the best way for us to troubleshoot this, but I understand if this isn't possible!
Red Gate Software Ltd.