Work rejected. Server says: Stale or alien proof of work
There will always be some stale work that is rejected by the server.

When there is a new block on the network (from any pool) we need to switch to building on top of this new block, at the new height. The server sends out new work data to all workers. But some of them are sending in work results at that moment, and it gets rejected because it is based on the old work data.

I would consider a reject ratio of 0.5% or lower to be normal and acceptable.