Syslinux - Feb 2018 - tftpd: write(ack): Operation not permitted

Dear list,
I'm having a randomly occurring issue with tftpd-hpa. One of the clients 
from time to time looses the possibility to access the tftp server. 
Neither put nor get is working when this happens. In syslog:

tftpd: write(ack): Operation not permitted

I increased verbosity level but nothing interesting was logged. When 
this issue happens the only way to regain connectivity is to either 
reboot the server machine (restarting just the service doesn't help) or 
changing the client's MAC address (IP is changed with it). What could be 
the problem?
-- 
Best regards,
Karol J?drzejczyk

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