(subject is filled with keywords so others can find this message. here
some additional keywords in german:
leere zwischenablage entleert sich kopieren einfügen kein text mehr da
zwischenablage verschwindet)

I've spent many hours and days(!) to find the reason why my clipboard
gets empty by itself. Sometimes it took few seconds, sometimes some
minutes. I was unable to do my work as software developer for weeks in
a reasonable way!

Now I've found the reason: It was Cloudmark Desktop!

http://www.cloudmark.com/desktop/releasenotes/
"Copy and Paste functionality - Fixed a problem where the Desktop
client could interfere with copy and paste functionality."

Thank you Cloudmark for not telling me that there is a new version.
Thank you for not warning about a bug which affects the whole machine.
Thank you for your so slow bugfixing. I will never use a software from
Cloudmark.