First, make sure remove all your offline devices from your account page, and perform a clean reinstall with these steps. If that doesn't help, can you let us know if you can use other Premium features such as ad-free listening?
Further, regarding the SD card, there are some things to think about when saving there:
SD cards should be formatted as portable and not as internal memory/adoptable storage. If it is the latter, reformat the SD card.
They should not be encrypted.
Once you've checked that, try a different SD card and see if it helps.