"The selected certificate path has errors: Not time valid"

[Andrea25905418whj6]

A friend of mine is having this issue on multiple of her signed PDF documents. Any suggestions on how to fix?

[MikelKlink]

When did she sign the document? 

The issuer certificate of her certificate was not valid anymore after 2021/09/15, so neither was her certificate. So if she signed thereafter, those signatures were invalid.