TFTP timeout problem during PXE boot

While setting up a PXE boot server on a CentOS 7 machine, I encountered a strange problem with TFTP. I cannot retrieve any files from the TFTP server without encountering a timeout problem. The boot process comes so far that I correctly get an IP address and a file name from the DHCP server. However, when the boot files are to be retrieved from the TFTP server, a "TFTP open timeout" message comes. If I manually make a TFTP connection to the PXE server from a local computer, I immediately get access to the server. But if I try with a "get pxelinux.0" command, I get another timeout message.
My firewall is set up correctly and it also makes no difference if I completely turn off the firewall. SeLinux is also disabled.
If I make a tcpdump on port 69 I get the following message:
12:34:33.477401 IP >  27 RRQ "pxelinux.0" octet tsize 0
12:34:35.481131 IP >  27 RRQ "pxelinux.0" octet tsize 0
12:34:39.490793 IP >  27 RRQ "pxelinux.0" octet tsize 0
12:34:45.477712 IP >  27 RRQ "pxelinux.0" octet tsize 0
12:34:53.441801 IP >  27 RRQ "pxelinux.0" octet tsize 0
12:35:03.384065 IP >  32 RRQ "pxelinux.0" octet blksize 1456
12:35:39.414843 IP >  32 RRQ "pxelinux.0" octet blksize 1456
12:36:51.422568 IP >  32 RRQ "pxelinux.0" octet blksize 1456
12:38:39.406732 IP >  32 RRQ "pxelinux.0" octet blksize 1456

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Unfortunately, the system log shows nothing useful:
Jan 15 13:13:19 tools xinetd[6993]: EXIT: tftp status=67 pid=7954 duration=0(sec)
Jan 15 13:13:21 tools xinetd[6993]: START: tftp pid=7955 from=
Jan 15 13:13:21 tools in.tftpd[7955]: no user tftp: Success
Jan 15 13:13:21 tools xinetd[6993]: EXIT: tftp status=67 pid=7955 duration=0(sec)
Jan 15 13:13:25 tools xinetd[6993]: START: tftp pid=7956 from=
Jan 15 13:13:25 tools in.tftpd[7956]: no user tftp: Success
Jan 15 13:13:25 tools xinetd[6993]: EXIT: tftp status=67 pid=7956 duration=0(sec)
Jan 15 13:13:31 tools xinetd[6993]: START: tftp pid=7957 from=
Jan 15 13:13:31 tools in.tftpd[7957]: no user tftp: Success

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The rights to the /var/lib/tftpboot directory are 0755.

Hope someone can help as I am getting more gray hair in the head.

Here are my setup file:

service tftp
        socket_type             = dgram
        protocol                = udp
        wait                    = yes
        user                    = root
        server                  = /usr/sbin/in.tftpd
        server_args             = -c -v -u tftp -p -U 117 -s /var/lib/tftpboot
        disable                 = no
        per_source              = 11
        cps                     = 100 2
        flags                   = IPv4

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Best regards.
Søren SjøstrømIT-Administrator/CEOAsked:
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how did you create the tftp user ?

If the tftp server is just used for PXE, then the -c and -U options are not required (and should not be used).

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