If, during a web design project, error messages during registration or login in WordPress no longer work with the Pie Register plugin installed, the first step should be to deactivate all other plugins in order to rule out problematic interactions.
If this does not restore error messages such as "Please enter a password" or "Username contains invalid characters", the registration or login process can be tested using the default theme as a second step. If this also makes no difference, all additional modules of the Theme My Login plugin should now be deactivated. In particular, the "Themed Profile" module can cause a loop in which the generated error messages are lost.
