I and at least one other RadRef librarian have found nothing at all on this protest. I searched Factiva and Lexis Academic (both are major newspaper databases, including major European papers), and New York Times fulltext (Proquest) including NY Times Historical back into the mid 1800s. I did very broad searches such as: balloon* and Rotterdam, pesticide* and spray* and balloon*, and pesticide* and balloon* and protest*; and got nothing close to any article about such a protest anywhere. I also tried Project Muse and JSTOR, just in case such an event was mentioned in a journal article, back to the early 1900s. Google Books gets nothing on it either. In case it was an event from the early 1990s to present, I also did proximity search: rotterdam n20 balloon* in the TX-All Text of Academic Search Premier and Masterfile Premier databases, but got only articles on balloon angioplasty, etc., with Rotterdam as a publisher or university address.
Possibly this happened and never got into the electronic databases, but it seems doubtful that ALL of these full text sources would have missed it. Even if government agencies or the mainline press had tried to suppress it, it seems odd that it would be reported only in that one source you mentioned.