I'm getting this error. i have 2gb of free space on my dropbox account and even when it was fuller it was never over..
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That's weird, the usage amount looks like it's returned some maximum value rather than the real value. That number comes from whatever the browser tells Construct is being used. So I guess it's a bug in the browser. You can probably just ignore it.
construct.net/en/forum/construct-3/general-discussion-7/wants-free-storage-even-393mb-173265
Another fairly recent topic that seems related and unaddressed. If it's reproducible, then a bug report on GitHub would help expedite a fix. I've never seen this myself though.
It's not clear that topic is related to this - it doesn't mention the same erroneous reading, and there are other reasons you may have a smaller quota than available disk space (e.g. private browsing modes often set a small quota).