Time Machine back cannot restore data warning. Is this probably legitimate? or start of ransomware?

Yes, this is a "normal" message you would get should the Time Machine backup set gets corrupted. This can happen for a number of reasons, but AFAIK, it doesn't happen due to ransomware.

Are these backups on an external drive or on a Time Capsule? Regardless, you will want to verify the backups integrity prior to having TM erase the backup.

How you do so will depend on where you TM backups are going to.

Old target hard disk? If so… I’d be inclined to start a second (new) external backup on a second (new) external hard disk drive, as a hard disk from 2015 may well be failing. Failing hard drives can start by getting slow, then start with corruptions. This also preserves what can be recovered from the current Time Machine archive, while creating a new backup on new storage.

To check the external hard disk failure data, you can use DriveDx: https://binaryfruit.com/drivedx

The failure data is not entirely predictive and not entirely reliable too, as some latent hardware faults can be missed.

2019 iMac running macOS Ventura 13.4.1 (c) Time Machine is from 2015.

Sorry, confused. Time Machine is software......it is part of the Mac OS Ventura operating system.

Might you really be asking about a Time Capsule.......hardware.....an Apple product that was used to store backups?

the idiot wrote:

i have the same issue however my computer gives me this error message but my wife's back-up, same time capsule, backs up fine. if i erase my back ups will it affect her back ups?

Yes, it will effect (erase) all backups on the storage, and if the target disk happens to be failing, the creeping corruptions will continue.

The Time Capsule disk will need to be erased to delete the corrupted Time Machine backup file.

Then, Time Machine will start all over again and move forward with new backups.

i have the same issue however my computer gives me this error message but my wife's back-up, same time capsule, backs up fine. if i erase my back ups will it affect her back ups?

thanks!

Time Machine make separate backups for each Mac. Deleting your Mac's backup set, will not affect hers.

thank you.

to be safe i added an new external drive the time capsule and started a new back up. i'm also trying Arq as a second level of backup for safety.

thanks again.