PuTTY 0.75 (or later) uses a new format for SSH private key (PPK version 3). PuTTY 0.74 or before can't read this format. As for PuTTY itself, this change is not a problem. However, this can be a problem for programs which use PuTTY internally. If the internal PuTTY version is not compatible with PPK version 3, the program can't use keys created with default setting of PuTTY 0.75.
If you see error message like 'Unable to load key file "*.ppk" (PuTTY key format too new)' in other programs, there must be the problem of this private key file format.
There are ways to avoid this issue. Also, FileZilla and other software will support the new format probably in the near future. You can wait for that time has come.
Private keys created with 0.74 can also be used in PuTTY 0.75. Please see below to create PPK version 2 private key with PuTTY 0.75.
1. launch PuTTYgen
2a. (click "Load" button to load existing private key)
2b. (If you create a new key, choose type and click "Generate" button (and then mouse cursor around). OpenSSH type public key text should be saved in a txt file.)
3. choose "Parameters for saving key files..." from "Key" menu
4. select PPK version 2 as below and then click "OK".
5. click "Save private key" of main window to save private key. (OpenSSH public key data won't be modified by this private key format change.)
Note: you can change PPK file version before generating/loading key.