I'm using cgminer 2.7.1 and also getting very high rejects for last few days. I changed to another pool but when I tried 50 btc again it was still having issues.
Here's what I see now:
Q:8439 A:37169 R:2051 HW:1 E:440% U:12.1/m
Up until a few days back I always had very low rejects and I have not changed anything at all.
Switching to another pool until this is resolved.
Here's a segment from my log and I've noticed this happens at each longpoll like this,
[2012-11-22 21:44:06] Accepted 137c3b45.db61c58d GPU 2 pool 0
[2012-11-22 21:44:12] Accepted 92371924.45e6083d GPU 1 pool 0
[2012-11-22 21:44:17] Accepted 2b38048d.c74af06a GPU 0 pool 0
[2012-11-22 21:44:19] Accepted a9f0dd71.97c68387 GPU 2 pool 0
[2012-11-22 21:44:21] Rejected 3eebb41e.e6e6decd GPU 1 pool 0
[2012-11-22 21:44:26] Rejected 9cf28f33.296b7b3e GPU 1 pool 0
[2012-11-22 21:44:27] Rejected 8e8d32ed.07f3dd80 GPU 1 pool 0
[2012-11-22 21:44:31] Rejected b59f4aa5.8c97759b GPU 1 pool 0
[2012-11-22 21:44:37] Rejected 5263f6f8.06c4b9e4 GPU 1 pool 0
[2012-11-22 21:44:44] Rejected c8333f3e.efc74e0e GPU 0 pool 0
[2012-11-22 21:44:45] Rejected f1bf2204.37ac6f20 GPU 2 pool 0
[2012-11-22 21:44:46] Rejected 1b1001fd.784bd629 GPU 1 pool 0
[2012-11-22 21:44:50] Rejected 5aa8f448.74c9068b GPU 2 pool 0
[2012-11-22 21:44:51] New block detected on network before longpoll
[2012-11-22 21:45:01] Accepted e05d9559.70fa6d66 GPU 1 pool 0
[2012-11-22 21:45:04] Accepted 63df1ca8.378aa886 GPU 1 pool 0
[2012-11-22 21:45:06] Accepted 8f568c87.a7ececb5 GPU 0 pool 0
Sometimes the string of rejects happens after the longpoll and not before. Sometimes only one reject but often many.
I have the same issue exactly as described above, except that I am using CGminer 2.8.7.