Strange Xfce window manager error

[Brian49]

During every Xfce session, I get many instances of this message in the .xsession-errors file:

"(xfwm4:2078): Gdk-WARNING **: Error converting selection from STRING: Conversion from character set 'ISO-8859-1' to 'UTF-8' is not supported"

I've reconfigured the locales on my system, to include both ISO-8859-1 and UTF-8, with UTF-8 set as the default (ISO-8859-1 can't be set as the default). This hasn't got rid of the error message.

I've searched on the internet and found some references to incompatibility between ISO-8859-1 and UTF-8, but no solution to the particular error on my system.

Can anyone shed any light on this, please? Many thanks for any kind help.