Installed 2nd Bitcoin client and invalidated blockchain progress
BU might have changed their default installation directory, but I doubt it. Then it sounds to me that you were running two copies of Bitcoin software working with the same blockchain data. Even when not running them at the same time, I'm not surprised that this might corrupt the block data. (If you've run them at the same time, no wonder at all, although I thought there was a lock in place to prohibit that.)


Don't run more than one Bitcoin software from the same directory.
Pick whatever flavor you may choose to run, install a current version of that.
Wait until synchronization is finished. Later blocks take longer than earlier blocks due to more content.