If you uninstall TheocBase the database remains in place. And the info in the database says you need to log in. That is why a reinstall does not work like this.
So, if (only if!!) you have never used TheocBase before (f you have, see next paragraph): do not uninstall the programme, but go to %appdata% in your windows explorer. Make sure the subfolder in AppData is Roaming, find subfolder TheocBase and remove theocbase.sqlite. Now open TheocBase and you will get a notification that a (new) database has been set up.
If you have used Theocbase before, you must not throw away the database. Instead make sure to download a database editor (like DB Browser, see my signature below). Navigate to the same file as mentioned earlier, and open it with the browser. In the second tab (Browse Data) choose the Settings table from the dropdown. In the 'name' column search for md5psw (or type that into the filter field) and change the corresponding entry in the 'value' field to 'false'. Then in the 'name' column also find 'login' and clear anything that is in the corresponding 'value' field.
[Carlos, you posted another question. I can see there is a question, but I cannot read it, since it is protected...]