5/2 Update - apparantly I’m not alone.
https://www.peercointalk.org/index.php?topic=4547.msg42853#msg42853
After upgrading to qt v0.5.3ppc-beta & refreshing chain on 4/28 and running fine since this AM noticed “WARNING: Invalid checkpoint found”. Getting the same message when folllowing the “getblockhash 234500” check mentioned here in Chat as way to verify checkpoint.
had to delete the blockchain and redownload, now the error is gone
You have these issues due to bad connectivity to other nodes on the correct chain. Network connectivity will improve as more nodes update their software and get on the correct chain. On https://bitinfocharts.com/ppcoin/nodes-active/ you can get IP addresses for Peercoin 0.5.3 and Peerunity 0.2.1 nodes. Add ten of them to ppcoin.conf, with addnode= in front of the IP, to get good network connectivity. Now delete the local blockchain before you start the client. After this you should not have any issues.
After culling a bunch of addnodes from these /Satoshi:0.6.3/Peercoin:0.5.3(v0.5.3ppc-beta)/ and re-downloading the chain my Qt client got stuck just below the published 234734 checkpoint with the “Checkpoint is too old… contact Developer” Error
So figuring it was time for Peerunity I culled a bunch of these /Satoshi:0.6.3/Peercoin:0.5.0/Peerunity:0.2.1(v0.2.1)/ installed Peerunity and am re-downloading but am not optimistic since Debug reports the number of blocks at 234714.
Will folllow now what appears to be the main thread on this issue that appears to be reaching critical mass with the community.
https://www.peercointalk.org/index.php?topic=4547.msg42853#msg42853
Btw - in my travels looking at explorers I found this one that groups the nodes by Version/Protocol (but presently they’re 10hrs behind)
https://chainz.cryptoid.info/ppc/#!network
Remember to delete everything in your datadir except wallet.dat and ppcoin.conf before you start re-downloading.