In the U.S., the rate at which mail in ballots were invalidated in 2020 was about 1.0% and it was about 1.4% in 2016 and 2018.
This is a somewhat tricky comparison, however, because the U.S. involves 51 different sets of laws governing when mail in ballots are valid, and there is wide variation from state to state (as shown at the link above) in invalidity rates for mail in ballots under the laws of different states, ranging from 0% to 6.42% in 2016 and from 0.34% to 7.56% in 2018 (only incomplete state by state data is available for 2020).
Some small percentage of ballots are also invalidated in in person voting, for a variety of reasons, such as selecting more than one candidate when only one vote for a post is authorized by law, or for writing in a candidate not certified as a write in candidate.
In contrast, in a Dutch election there would be only one applicable statute.
One pertinent distinction is that the Dutch election process would apply only to voters over the age of 70 years of age, while the U.S. figures, under U.S. election laws, apply to voters of all ages, or even when age or disability is a condition to absentee ballot access, to a much broader age range than is allowed in the Dutch election.
It is not unreasonable to guess that people over the age of 70 in the Netherlands who elect to vote by mail rather than to vote in person, as is customary there, are disproportionately suffering from serious physical or minor cognitive disabilities (even within the subset of potential voters age 70 or above) and that this highly biased set of mail-in voters contributes greatly to the invalid ballot rate.
It is also likely that since mail in ballot is not the historical norm in the Netherlands, that officials have not used particularly clear instructions and forms to communicate to mail in voters what they must do, something that could be addressed with the better mail in ballot process and ballot design found in jurisdictions where mail in ballots are more common.