The latest version 3.7.8-8.7 x64_68 provides an error message when starting up claws an error message is displayed. Hitting close does start Claws, though my addressbook is empty and doesn't contain all my addresses.
Could not read address index
is ~/.claws-mail/addrbook/ writable?
That error is related to a missing addrbook--index.xml, which would normally get re-created automatically.
Anyway, this is not a bug, but related to your setup.
It is writable, date changes after starting claws. I noticed a bunch of old .bak files. Removed those and now I have an empty address book and no error, but not my addresses.
I got the same message. Symptoms: Address book empty, and addrbook--index.xml is 0 bytes.
After removing the 0 byte file, it gets properly recreated and the message does not appear anymore.