There are several settings for events...
"event1=1" needs to be used with a numeric or currency (not "counter") event
You need to make sure that your event is not set to "counter", as counter is not expecting a value to be passed.

Also, are you sure you are using "Serialized" event? Or are you using "Record Once per Visit"?

Serialized refers specifically to the "Use Event ID", where you pass a unique serial number to the event (i.e. "event1:12345"), and anything with a matching id, within the history of tracking will be ignored as a duplicate. If Adobe sees "12345" again, it won't count the event ever again, from any user.
If you have a serialized numeric event, you would need to pass both the value to increment the event on, as well as a unique serial number (i.e. "event1:12345=1")
From the context of your example, I am assuming that you are trying to use a numeric incrementation, using the standard "Record Once per Visit" logic (not serialization).
I suspect that you haven't configured the event to be numeric, and that is causing issues with Adobe trying to process the information, resulting in the event being thrown out as invalid.
Can you double check the setup of your event, and ensure that the event is set up to receive a passed value for incrementation?