Why does PicketLink based SP fail to allow logins and repeatedly log a "PLFED000203: Assertion has expired" error message?

Issue

Why does our PicketLink based SP fail to allow logins and repeatedly log a "PLFED000203: Assertion has expired" log message?

It appears to go into a loop logging the following error:

11:05:45,603 INFO  [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) PLFED000203: Assertion has expired with id=ID_bf472f3b-12cc-4152-9b75-f4f6b1806b8d
11:05:45,610 ERROR [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) Assertion has expired. Asking IDP for reissue
11:05:45,681 INFO  [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) PLFED000203: Assertion has expired with id=ID_ffe17d07-cc07-4dd3-b302-1e221cf63b85
11:05:45,682 ERROR [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) Assertion has expired. Asking IDP for reissue
11:05:45,750 INFO  [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) PLFED000203: Assertion has expired with id=ID_d358853f-c76f-47ed-8004-e830719644be
11:05:45,751 ERROR [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) Assertion has expired. Asking IDP for reissue
11:05:45,819 INFO  [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) PLFED000203: Assertion has expired with id=ID_dba9fbcb-e0b6-4536-b187-b50969b9d33e
11:05:45,826 ERROR [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) Assertion has expired. Asking IDP for reissue
11:05:45,899 INFO  [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) PLFED000203: Assertion has expired with id=ID_f74ec6d8-79a7-4e15-bab5-5072e7503715
11:05:45,900 ERROR [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) Assertion has expired. Asking IDP for reissue
11:05:45,978 INFO  [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) PLFED000203: Assertion has expired with id=ID_382fa49a-16d6-4e92-8a90-404df9da3a93
11:05:45,985 ERROR [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) Assertion has expired. Asking IDP for reissue
11:05:46,057 INFO  [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) PLFED000203: Assertion has expired with id=ID_09a2db1c-ca4f-40b6-8ce7-e86b4adeeccb
11:05:46,057 ERROR [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) Assertion has expired. Asking IDP for reissue
11:05:46,113 INFO  [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) PLFED000203: Assertion has expired with id=ID_2d22a90e-6516-4130-8694-d638d45641db
11:05:46,119 ERROR [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) Assertion has expired. Asking IDP for reissue
11:05:46,176 INFO  [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) PLFED000203: Assertion has expired with id=ID_74986882-654e-4534-8ccf-b6ddda344520
11:05:46,177 ERROR [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) Assertion has expired. Asking IDP for reissue
11:05:46,215 INFO  [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) PLFED000203: Assertion has expired with id=ID_9ef3cada-463f-4bfb-9086-0b20ae84f360
11:05:46,216 ERROR [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) Assertion has expired. Asking IDP for reissue
11:05:46,254 INFO  [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) PLFED000203: Assertion has expired with id=ID_9876aab6-4f8e-424a-9bbe-f9dc405175d1
11:05:46,254 ERROR [org.picketlink.identity.federation] (ajp-/192.168.1.103:8009-1) Assertion has expired. Asking IDP for reissue