Ok, I've caught it rebooting. Here is the log in question just before, and just after:
39802183mS H323Evt: Recv: RegistrationRequest XXX.XXX.XXX.39; Endpoints registered: 92; Endpoints in registration: 0
39802930mS H323Evt: Recv: RegistrationRequest XXX.XXX.XXX.23; Endpoints registered: 92; Endpoints in registration: 0
39803887mS RES: Wed 19/8/2015 18:42:28 FreeMem=50155412 49381900(2) CachedMem=773512 CMMsg=6(7) Buff=5200 1416 1000 12364 5 Links=21689 BTree=0 CPU=51/189/3933/12555/18569/1
39803888mS RES2: IP 500 V2 8.1(95) Tasks=53 RTEngine=0 CMRTEngine=0 ExRTEngine=0 Timer=242 Poll=0 Ready=0 CMReady=0 CMQueue=0 VPNNQueue=0 Monitor=3 SSA=1 TCP=200 TAPI=0 ASC=1 SYS=MNTD OPT=UMNT SDSPD=2034
39803888mS RES4: XML MemObjs=45 PoolMem=2097152(1) FreeMem=2082064(1)
39806669mS H323Evt: Recv: RegistrationRequest XXX.XXX.XXX.100; Endpoints registered: 92; Endpoints in registration: 0
39806854mS H323Evt: Recv: RegistrationRequest XXX.XXX.XXX.23; Endpoints registered: 92; Endpoints in registration: 0
39807472mS H323Evt: Recv: RegistrationRequest XXX.XXX.XXX.87; Endpoints registered: 92; Endpoints in registration: 0
39808643mS PRN: Alarm log cyclic wrap
39808643mS PRN: Begin Stack Trace
39808644mS PRN: pc=f01bf568
39808644mS PRN: lr=f03562bc
39808644mS PRN: findfunc f01cc0dc f01bf950 f0dd4be4 f03581d4 f030b200 f030b1ac 00000000 00000000 00000000 00000000
39808644mS PRN: End Stack Trace
39808644mS PRN: ScheduleHistory NOT enabled: Task = MemoryCardMonitor File = ../platform/platform.cpp Line = 1186
39808644mS PRN: .FATAL FileSystemSMXFSTask::GetDriveNum: Bad d address=f03562bc d=5 pc=f01bf568 f01cc0dc f01bf950 f0dd4be4 f03581d4 f030b200 IP 500 V2 8.1(95)
********** contact lost with XXX.XXX.XXX.4 at 18:46:23 19/8/2015 - reselect = 1 **********
******************************************************************
********** SysMonitor v10.1 (63) **********
********** contact made with XXX.XXX.XXX.4 at 18:47:31 19/8/2015 **********
********** System (XXX.XXX.XXX.4) has been up and running for 26secs(26326mS) **********
********** Warning: TEXT File Logging selected **********
********** Warning: TEXT Logging to File STARTED on 19/8/2015 18:47:31 **********
26326mS PRN: Monitor Started IP=XXX.XXX.XXX.203 IP 500 V2 8.1(95) MedFusion
(IP Office: Supports Unicode, System Locale is default)
26327mS PRN: LAW=U PRI=2, BRI=0, ALOG=4, VCOMP=42, MDM=0, WAN=0, MODU=1 LANM=0 CkSRC=0 VMAIL=0(VER=0 TYP=1) 1-X=0 CALLS=0(TOT=0)
26536mS RES: Wed 19/8/2015 18:43:52 FreeMem=56559868 56425892(1) CachedMem=133976 CMMsg=2(3) Buff=5200 1436 1000 12381 5 Links=28461 BTree=0 CPU=0/0/4294967295/0/0/0
26536mS RES2: IP 500 V2 8.1(95) Tasks=45 RTEngine=0 CMRTEngine=0 ExRTEngine=0 Timer=100 Poll=0 Ready=0 CMReady=0 CMQueue=0 VPNNQueue=0 Monitor=2 SSA=0 TCP=15 TAPI=0 ASC=1 SYS=MNTD OPT=UMNT SDSPD=2034
26536mS RES4: XML MemObjs=0 PoolMem=0(0) FreeMem=0(0)
27220mS PRN: A:\system\ws already existed with attr 10
27221mS RES3: Tasks=(6227)StartUp (69)Daemon (22)FileSystemSMXFS
27221mS RES: Wed 19/8/2015 18:43:52 FreeMem=54250412 54119204(1) CachedMem=131208 CMMsg=2(3) Buff=5200 1433 1000 12364 5 Links=27913 BTree=0 CPU=467/583/478802/478802/478802/0
27221mS RES2: IP 500 V2 8.1(95) Tasks=51 RTEngine=0 CMRTEngine=0 ExRTEngine=0 Timer=95 Poll=0 Ready=0 CMReady=0 CMQueue=0 VPNNQueue=0 Monitor=2 SSA=0 TCP=15 TAPI=0 ASC=1 SYS=MNTD OPT=UMNT SDSPD=2034
27222mS RES4: XML MemObjs=45 PoolMem=2097152(1) FreeMem=2082064(1)
27222mS PRN: Config Write Wake Up
27290mS LIC: INFO: 1 power user licenses 0 0
27292mS PRN: Created Tone MOH Source 1
27292mS PRN: Playing tone MOH Source 1
27293mS PRN: A:\system\ws\up already existed with attr 10
27294mS PRN: A:\system\ws\br already existed with attr 10
27298mS PRN: System Started from System Card, Primary Directory
27299mS PRN: WARNING:
27299mS PRN: System was not Shutdown correctly
27299mS PRN:
27299mS PRN: Last Shutdown/Reboot: Wed 19/08/2015 07:38:29