An old thread, this one, but it helped me and maybe I could add something. Basically, I think it's solved
First, my situation was the same: as soon as I added my own SymolSource to hivemind.SymbolSources tapestry refused to start complaining about missing keys (e. g. org.apache.tapestry.template-encoding).
The described workaround, to define the keys in the Application file worked also for me.
However, in the end my own SymbolSource seems to be the root cause. It is based on ResourceBundle. ResourceBundle.getString throws an exception if the key is missing, whereas a SymbolSource should return null for missing keys. Catching the exception in the SymbolSource implementation seems to fix this completely (besides the obviously strange error message from Tapestry).
