What size is the tank? What's the temperature in the tank and what are you using for a heat source? And how humid is the air outside of the terrarium? If the temp is too low or if the humidity is really high outside the tank it could affect the humidity inside the tank. Also, what are you using for substrate and do you have real plants in there? Posting a picture of your vivarium would be very helpful.
White's tree frogs prefer humidity around 60% so that is high for them. I'm not sure what health problems could be caused by it if any, but it could stress them. It will also promote mold growth inside the vivarium which can harm the frogs. I'm sure if we get enough details we can help you get that humidity down in your tank

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Oh and by the way, Welcome to the forum
