Been trying to enter a game I saw elsewhere online to analyze in Fritz, turned off engine and all is fine until the time came to castle, just get the dreaded 'whistle' informing of an invalid move. How do you get around this? Thank you,

I would guess it your position, not Fritz.  Either you have previously moved the K or R, or you are trying to castle across a line of check.

No, I've checked (no pun intended), it's a game where white was castling after only 4 moves, not in check or across check and the king and rook haven't budged. Thanks for your reply.

This is the opening: 1.e4 e5 2.Nf3 Nc6 3.d4 exd4 4.Nxd4 Ne5 5.Be2 d6 6.O-O. After 5 moves not 4 as I said earlier.

Reboot F13. If still no go, reboot your PC. If that doesn't help I'd contact whoever you bought it from for assistance, or try this clumsy but often effective fix: (which does have the advantage of cutting out all the trouble-shooting) uninstall and re-install the entire program (all your games should be left intact since they're stored in another directory, but I suggest you double-check here before uninstalling)

BTW my suggested fix works for all sorts of programs, not just F13. I'm running F12 myself BTW.

Many thanks.

Works fine on my Fritz 13.  Have you gone back into the

File-->Activation-->Update Program and check to make sure you are using the latest version?  When I bought my copy it was build 5, now it's up to either 13 or 15 (forget which) so there have been numerous and frequent updates that may fix it.