I get "Internal Server Error"

Neither works:

SELECT ?author WHERE {
  ?author ^wdt:P50 / wdt:P31 wd:Q13442814 .
}
ORDER BY RAND()
LIMIT 1

Two SPARQL queries after each other.

SELECT (COUNT(?author) AS ?count) WHERE { [] wdt:P50 [] }

?offset = ?count * RAND()

SELECT *
WHERE {
  [] wdt:P50 ?author
}
OFFSET {{offset}}
LIMIT 1

Here the bad news is that the last query may return something like "t1105721757" for ?author

Such "t1105721757" results usually indicate cases where a statement has things like "unknown".
I suppose this is a bug in the serialization - not sure whether there is a Phabricator ticket for this.