It seems like this is related to passivation, do any of your observations agree with this or suggest something else?
Is there any behavior in your application that you believe is associated with this error?
A complete log file is generally helpful to review with any log errors.
It doesn't seem like this is a known issue in Pega 7.3.1, so you may want to open an SR for us to look into it more deeply if you don't want to share a complete log here. Be sure to update this post with the SR number if you do open one.