There are no troubles in the functions of guacamole. But there are some log entries in our syslog file:
hostname kernel: [790049.145417] guacd: segfault at 78 ip 00007f70a34fee99 sp 00007ffe7fda2b50 error 4 in libguac-client-vnc.so.0.0.0[7f70a34f4000+f000]
hostname guacd: Client disconnected
The SSH and VNC connections are working without any problems.
It seems that the segfault occurs on a client disconnect.