Block #338819 deemed invalid by v0.15.1
That error means that the block got corrupted somewhere so it became invalid. The only way to fix this is to re-download the block.

To do so, first stop Bitcoin Core. Then go to the Bitcoin Core data directory and then the blocks folder within that. Delete the highest numbered blk*.dat and rev*.dat files. Then start Bitcoin Core again. When it asks whether you want to reindex the databases, choose yes. Bitcoin Core will now rebuild the block indexes and then continue with syncing once it is done. Note that while it is reindexing, it will look like it is syncing the blockchain again, but it is not. It is using the data on disk to build the block index and chain state until it needs to download blocks again to continue syncing.