crashed with lots of error response "authentication needed: password or unlock"

Our ethereum network node crashed on oct 24, with lots of error response "authentication needed: password or unlock". The QPS is about 1~2, which is very low. According to our ab test, the node can support at most 60 QPS invalid request.

here are logs

WARN [10-24|18:29:47.975] Served eth_signTransaction               conn=45.76.106.157:6362    reqid=84   t=98.264µs    err="authentication needed: password or unlock"
WARN [10-24|18:29:48.062] Served eth_signTransaction               conn=144.48.63.243:50830   reqid=51   t=64.547µs    err="authentication needed: password or unlock"
INFO [10-24|18:29:48.224] Setting new local account                address=0x40A49838dd7f61666258254a2D9f611eFC408800
INFO [10-24|18:29:48.232] Submitted transaction                    fullhash=0x699e9c6d24198c0bff1edf147ee133eb6c2acc326bf8e1becff635b2035b6a78 recipient=0x2E65E12b5f0fD1D58738c6F38dA7D57F5F183d1c
fatal error: runtime: out of memory
runtime stack:
runtime.throw(0x10e662b, 0x16)
    /usr/lib/go-1.11/src/runtime/panic.go:608 +0x72
runtime.sysMap(0xc47c000000, 0x10000000, 0x23bc178)
    /usr/lib/go-1.11/src/runtime/mem_linux.go:156 +0xc7
runtime.(*mheap).sysAlloc(0x238d3e0, 0x10000000, 0x680031f220, 0x7fe5bdffacd8)
    /usr/lib/go-1.11/src/runtime/malloc.go:619 +0x1c7
runtime.(*mheap).grow(0x238d3e0, 0x8000, 0x0)
    /usr/lib/go-1.11/src/runtime/mheap.go:920 +0x42
runtime.(*mheap).allocSpanLocked(0x238d3e0, 0x8000, 0x23bc188, 0x0)
    /usr/lib/go-1.11/src/runtime/mheap.go:848 +0x337
runtime.(*mheap).alloc_m(0x238d3e0, 0x8000, 0x101, 0x149c0f8)
    /usr/lib/go-1.11/src/runtime/mheap.go:692 +0x119
runtime.(*mheap).alloc.func1()
    /usr/lib/go-1.11/src/runtime/mheap.go:759 +0x4c
runtime.(*mheap).alloc(0x238d3e0, 0x8000, 0x7fe5bd010101, 0x43ada0)
    /usr/lib/go-1.11/src/runtime/mheap.go:758 +0x8a
runtime.largeAlloc(0x10000000, 0x460101, 0xc46c000000)
    /usr/lib/go-1.11/src/runtime/malloc.go:1019 +0x97
runtime.mallocgc.func1()
    /usr/lib/go-1.11/src/runtime/malloc.go:914 +0x46
runtime.systemstack(0x0)
    /usr/lib/go-1.11/src/runtime/asm_amd64.s:351 +0x66
runtime.mstart()
    /usr/lib/go-1.11/src/runtime/proc.go:1229

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