"Unknown" Spill Range when it is very much known

I am trying to filter a list.

Currently the formula looks like this:

=FILTER(BS1#;DZ1#;"empty")

BS1# is simply a list of things with several attributes, the very first column being a boolean on whether to include the row.

DZ1# is simply the first column of BS1#.

Both are always the same dimensions and never throw out any errors, however the above FILTER randomly gives a #SPILL error, always saying it does not know how large the spill area is. There is no pattern to when this error is given.

While BS1# has a lot of OFFSETs in it and a previous version of that did show the same random error behavior. I fixed that one and it is now always the same size. I have no idea why the FILTER is doing this.

In cases where it does show an error and I go into the cell, mark the entire formula and mouse over, it does not give the error and instead gives the array in text as though there is no error!!

I am running the 64bit version and my RAM is at 30% usage, but EXCEL is only at 382 MB, far less than Google Chrome.