This is a Javascript error when you can't see your own text in Yahoo Messenger. To fix the error, you will need to re-register the DLL files. You might try other fixes, but I can say that the below instructions help me as well.
Firstly, uninstall Yahoo Messenger and Java, then download and install CCleaner. Clean the registry items and the temporary or other files that are not in use anymore. Reboot the PC and download the packages again (Messenger & Java). Install them and access Start > Run > iexplore. Go to Tools > Internet Options > Advanced> Reset
After this process, go to Start > Run and type the following commands:
regsvr32 jscript.dll
and then the next one. You should see a Successful message after pressing enter.
regsvr32 vbscript.dll
Restart Yahoo Messenger and type something and you will see it will work correctly.