This issue is the result of missing silicone. You should apply waterproof sealant to all four corners of your pergola. The silicone should fill the corner piece until it is overflowing as outlined in the manual. If you follow this link and select your pergola, you will find more information in detail: https://www.pergolux.app/assembly and you can also watch this on our YouTube channel: How to Apply Silicone
If you've already applied silicone, take it out and reapply as described (see picture).