InfoPath Dev

I'm somewhat of a noob when it comes to InfoPath and I really need some help with this one. I just started this role a few months ago and inherited a lot of InfoPath forms that I did not design. I know that this error is due to a data connection pulling too much data. I can see that the form has several data connections to other SharePoint lists. This error just started today and was working fine previously, so I suspect one of the lists that the form is pulling from eventually hit a size that is generating this error. I don't know where to turn next as far as resolving this. There is a dropdown at the top of the form which is currently blank due to this problem. I need some help figuring out how to modify the data connection, perhaps to filter it and somehow avoid this error. Or do I instead need to explore deleting some items from the secondary list? The list which the blank dropdown pulls from has 3300 items. Again, I am very new to InfoPath so any help is appreciated.

On SharePoint Online, we can't increase this limit. So the form has to be modified to not return all the data from any list or library with greater than 5,000 items. The first thing I usually do is look at all the data connections and see which ones are set to run on form open - any one of those could be your trouble. Find any SharePoint list or library the form queries and check how many items are in each.