iMac freezes in sleep mode and does not awake anymore, must be booted

Hi everyone,

my iMac (OS Mojave 10.14.4, iMac late 2013, 29.Ghz Intel Core i5, 8GB 1600 Mhz DDR3) since yesterday repeatedly freezes in sleep mode and I am not able to wake it up, not with a mouse click, neither with keyboard, or with the on-of button.

The only way out has been the first time to take away the power supply, the second time to press the button long enough to cause a new boot. Hardware test does not find any issue.

When rebooted apparently everything is working fine.

Thanks a lot for any advice!

Regards

Claudio