QGIS: No values in field after creating spatial index

I want to join a vectorlayer with a grid using the algorithm "join attributes by location (summary)". It took hours to execute, but it worked. Then I found This thread which told me I have to create a spatial index for the layer first.
So I did that for the vector layer, but after executing the algorithm "join attributes by location (summary)" again, all values in the desired field were zero? It took only 2 seconds to execute, but there were no values? What could be the reason for that?

Update:
I thought I found what was the issue: To speed up the process, I deleted all fields except my value field. There must be an ID-Field which acts as the spatial index though, so my fault was to delete this one as well. So I did it again, leaving this field there, and it worked! But it only worked once, all tries after that executed right away, but once again, I got no values, just zeros…