Looks like you hit some software bug (there are several similar bugs in the bug toolkit).
The whole 7.6 software train was deferred by Cisco (because of bugs and such) and should not be used. I suggest to upgrade to 8.2.151.0.
This was fixed in 7.6.130.0 and newer (it's just a small selection of the full list):
CSCuq36902
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Memory Leak with Internal webauth in a loop for longer period
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CSCuj05274
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WLC Crash - Reaper Reset: Task “”loggerMainTask”” missed software watchdog
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CSCuo86819
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WLC crashes in 7.6.120.0 - memory corruption caused by Webauth
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So yeah, I stand with the upgrade to 8.2.151.0.