Good day one and all,
I ran the trimhistory command as root, which I realize afterwards, probably caused this problem - confirmed by messages of yore in this mailing list.
However, restarting xymon / setting permissions on contents of history files (/var/lib/xymon/hist/) to xymon:xymon doesn't seem to help.
The rest of Xymon is working, /var/lib/xymon/hist/allevents shows current changes.
I do see the following in /var/log/xymon/history.log:
2016-03-16 11:51:05.989979 xymond_channel: Peer not up, flushing message queue
I make use the latest Xymon release from Terebathia on CentOS 7, Xymon 4.3.26-
1.el7.terabithia.
Any suggestions would be greatly appreciated.