Hi I am having the same problem, the key that came with my address provided through Rippex desktop wallet appears to include only valid characters listed above but it continues to tell me that the key is invalid and I can't open it elsewhere or create a new wallet within the desktop app. Receiving the same response on the minimalist client, this is the key I was provided with when prompted to check, I wrote it down twice independently of one another to make sure. Was there any further thoughts here?