What might be causing this, any ideas?
Server: Firebird 64bit classic on Ubuntu Linux (2.5.4.26856)
Client: Windows 10

After this error, the connection drops & client app hangs.

Mar 24 15:05:58 kernel: [2481720.238260] fb_inet_server[10098]: segfault at 0 ip 00007f5cba31bb05 sp 00007f5cb86d4b90 error 4 in libfbembed.so.2.5.6[7f5cba1bf000+4cd000]>>

#define EINTR 4 /* Interrupted system call */
You need to capture a coredump on the server and analyse it.
https://www.ibphoenix.com/resources/documents/search/doc_36

regards
Paul