There is absolutely no rule regarding the sequence of such diagrams.
Sometimes, when the data structure and behavior of your domain model is easily defined or well documented, creating class diagrams first provides clearer abstractions that help create a sequence diagram that makes sense.
In other cases, when the nature of the domain model is unknown or unclear, it will make sense to first create a sequence diagram and then extract classes from it.
I am sure that revisions of these diagrams will become coincident with each other (for example, sequence diagrams can display what was not taken into account in class diagrams, and vice versa).
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