This can have several causes. One is an outdated browser cache. Delete this if necessary.
Furthermore, an invalid character can also lead to an incorrect JSON being generated by the backend. Since JSONs must correspond to a specific schema and many parts in Pi.Alert exchange data in JSON format, this can lead to errors. The "Test Main JSON Calls" function is available for this purpose under Maintenance -> Settings -> Advanced. If you report an issue in this regard, please include the output of this function.
- rename the still existing "pialert" directory e.g. to "pialert-old"
- reinstall Pi.Alert
- copy the database from "pialert-old/db" to "pialert/db/" and overwrite the existing database there
- copy "pialert.conf" from "pialert-old/config" to "pialert/config"
- execute the command "./pialert-cli set_permissions" in the "pialert/back" directory
- now everything should work again