I ran btrfs check without --repair but apparently it fixed some issues and now the rebalance is continuing without exiting...
# btrfs check /dev/sdc Opening filesystem to check... Checking filesystem on /dev/sdc UUID: d9a2a011-77a2-43be-acd1-c9093d32125b [1/7] checking root items [2/7] checking extents [3/7] checking free space tree cache and super generation don't match, space cache will be invalidated [4/7] checking fs roots [5/7] checking only csums items (without verifying data) [6/7] checking root refs [7/7] checking quota groups skipped (not enabled on this FS) found 4032094437376 bytes used, no error found total csum bytes: 3933617376 total tree bytes: 4070244352 total fs tree bytes: 22151168 total extent tree bytes: 24248320 btree space waste bytes: 54106861 file data blocks allocated: 4028024193024 referenced 4028024193024
Thanks, Richard