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