I support over 100 franchises who use QBO and converting sub-customers to projects is a necessity in our organization.  I've received over a dozen calls recently from offices who have all started getting this error.  It is not a cache/cookie issue as I have tried to convert their sub-customers on my computer and get the same error and I've cleared my browser.  When these offices contact QBO support they get absolutely NO help.  It feels like the people on the other end of the line have absolutely no idea what is going on and are unwilling to figure it out.  It's so frustrating.