After installing DeveloperOne Power Calendar on my I-Mate SP3i and restarting the phone, it did not want to start anymore. The I-Mate screen came on, followed by the Windows Mobile screen, followed by a new restart. I cannot say it is due to the Power Calendar software because I also installed some other software before (like the Opera Technical Preview). So one of those apps could have caused it.
The only way out was a hard reset but that did not turn out to be simple. In the end, I found the following instructions on the ModaCo site for the SPC C500. This also works for the I-Mate SP3i. Now, I think I will install Sprite Backup on my SmartPhone to avoid having to reinstall all the apps on the phone from scratch. :-)
