Had this problem for some time, got it sorted last week.
In my case the problem was with the cd, spoke to Lexus Manchester recommended a new sat nav reader for £2.3k, spoke to Lexus Stockport confidently advised that its the cd told me to pop in and check system and use new cd.
All is working A1, went to London on Friday worked a treat.
So appears it was not the reader but just the cd, someone has some explaining to do, in process of compiling letter to Lexus Manchester for explanation.
So people try the upgraded cd first it could resovle the issue, as the box very rarely goes faulty.