libcurl 8.11.1 works, libcurl 8.12.0 gurus

I'm posting this here in the hope somebody can help and maybe olsen will see it.

The NetSurf toolchain was recently upgraded to libcurl 8.17. Now when NetSurf starts to do some network access the machine crashes with guru 8000000B. I've tracked it down to curl_multi_perform in libcurl 8.12 or newer.

libcurl is built against AmiSSL and weirdly the curl executable seems fine. There were some changes in libcurl 8.12 related to OS3 bsdsocket but as far as I can tell everything is sane.

I've tried both Miami and Roadshow so it doesn't appear to be the TCP/IP stack.

NetSurf and libcurl built against AmiSSL on OS4 is working correctly.

I've run out of things to try, can anybody suggest anything which might help?