Theme "desactivated" without doing anything

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Hi

We have few C5 website created and hosted for our clients, and only one has this problem.

For some reasons, sometimes, without anything done on the website, everything doesnt display properly. I've realised that going to dashboard>pages>themes will let me activate the theme (which was activated before)

Do you know any reason why a theme would be desactivated automatically?? the problem is that we apparently lose the custom attributes...

Any input welcome

Thanks