ABAP Web Service call leading to Error 503

Hello,

We have a service which is called from outside SAP.

The calling system spawns 5 threads and the service is called by each thread.

The process runs fine for some time and then goes into error 503 'Service Unavailable'.

The log seen on ICM is :

[Thr 09] PlugInStopConn: role: Server(1), error: -12, write_err: 1

[Thr 09] PlugInStopConn: write error response

[Thr 09] HttpPlugInWriteErrorText: return static error for "No more connections available" (-12)

[Thr 09] HttpPlugInWriteErrorText: templ_path: /usr/sap/DCI/DVEBMGS00/data/icmandir/error_templ, err_idx: -1

[Thr 09] ICT: IctIHttpOpenMessage: 107c41f00 typ=2

[Thr 09] HttpParseResponseHeader: no transfer-encoding set

[Thr 09] HttpParseResponseHeader: Version: 1000

[Thr 09] HttpParseResponseHeader: Keep-Alive: 0

[Thr 09] HTTP response  [2/355484/1]:

[Thr 09]   HTTP/1.0 503 Service Unavailable

[Thr 09]   date: Fri, 01 Jul 2016 00:21:46 GMT

Any expert tips and pointers will be highly appreciated.