Found "chain" folder under Library -> Application Support -> Bitshares X and deleted it. Now redownloading the whole blockchain.
It went from "catching up ..." to "Not connected" immediately when it was at approx. at block 554891. Now it is stuck at 554891 again.
I made a transaction during the first blockchain download beforehand at maybe the bock 554891, when the chain wasn't fully downloaded. Could this be an issue? Should I 'delete' the pending transaction in my wallet too and maybe solve the problem this way? How can I delete the transaction?
Ok, I found a backup of my wallet without the pending transaction. Redownloaded the client, deleted the old folders. Now waiting for block redownload.