Let me take a look at the startup code and see if I can see anything.

@HelmutWe, (mouser is the software author) he's saying he will check out the Screenshot Captor code just in case the startup problem is its fault

What does the error message say?

SC doesn't care about differences in screenshots -- but SC will by default complain if you capture an entire image that is only one color.. This is a check to make sure something hasn't gone wrong. You can disable this option here in the "Miscellaneous Tweaks" tab:

@mouser: It took me quite a time to find out that I had to run the setup as admin. As my PC-dealer told me I´d have all possible rights on my private PC that didn´t occur to me. Anyway, there is no problem anymore. Whether to decide if this is a case of "problem solved" or "problem vanished" I don´t know. And the autostart-option takes some seconds before it is recognizable. That is i have to try some screenshots before I hear the meantime familiar click sound.
@Kodezwerg: Dein Hinweis "Your Windows Admin Account may differ from Logon Account" war eigentlich goldrichtig. Ich habs nur nicht kapiert, weil mein Händler mir sagte, ich hätte alle Rechte. Ja, es gibt einen Unterschied zwischen Window Account und Logon Account. Bedauerlicherweise weisen weder Acronis noch Avast auf sowas hin.