You should try to keep atlas and webapi at the same version. We work to make things backwards compatible (such that atlas 2.10 can talk to webapi 2.11). However, it is usually not possible for Atlas 2.11 (newer) to talk to WebAPI 2.10 (older) because something in Atlas 2.11 will depend on an end point in WebAPI 2.11 that wasn’t available in WebAPI 2.10.
Achilles is slightly disconnected from WebAPI/Atlas, but I think there’s a dependency on Achilles tables such that in order to get the ‘record count’ on vocabulary search, we read from the Achilles tables when WebAPI starts up. However, I don’t think that would lead to a service failure if the tables are not there. Rather, you just won’t see record counts (RC/DRC) on the vocabular search screens.
Is it the current state that you have Atlas 2.11.1 installed, but pointing to a WebAPI that is not up to version 2.11?