As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE FLIGHT AT FL380 AUTOPILOT "B" DISENGAGED WITH NO PILOT INPUT. IT WAS SMOOTH FLIGHT CONDITIONS IN THE CLEAR. AUTOPILOT "B" WAS REENGAGED WITH NO FURTHER ISSUE. THE AIRCRAFT WAS TRIMMED AT THE TIME. GROUNDED - YES. PERFORMED B737-800 FIM 22-11 TASK 801, DFCS BITE PROC INCLUDED AUTO, INTERACTIVE & SURFACE TESTS. NO FAULT NOTED. OPS NORMAL.