This message is new in BOINC 5.8. It means that there isn't enough free RAM to run the task, according to your preferences.

But the default is only to use 50% of RAM while your computer is in use. This is a significant change from earlier versions, and many WCG users will find that they no longer have enough memory. But don't worry: you can change it back.

To fix it, go to your device profile.

• Select "Custom Profile".
  
• Check that "Use no more than: X% of total memory" is set to 100%.
  
• Save the profile, even if it was already at 100%.
  
• Update BOINC using the Update button in BOINC Manager. Make sure you are not using local preferences.
  

Doing this adds the new preference to your settings, and will override the BOINC default. If you have additional device profiles, you will have to repeat these action for each profile.