When I try to allocate/Assign a person to a work task from the prepare work order window (Assign multiple Task > Type Resource > Select Resource from Drop down> Click on OK) at times it get below error message (picture 1) which results in below assignment without a resource tied to it (picture 2) – that cannot be used and need to be cancelled. This happens intermittently and doing the exact same (typing the same letter, pressing the same resource) works 1 minute after.
Pic 1:

Pic 2: