You should try different things, and provide much more details, such as :
- backup without logs
- backup without non standard plugins
- Do you use non standard plugins on your Moodle (and in these courses)?
- Do you use big ressources (videos)?
- Do you have lots of users, with lots of activities?
- When does this error message occurs (at the beginning, after a certain time...)?
- if they previously backuped well, what changed recently on these courses?
- Where is your Moodle hosted?
- did something change recently on you Moodle (hosts)?
- What are the limits of execution time?