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When merging layers I get following message:

Error executing algorithm Merge lines {} field in layer {} has different data type than in other layers.

What am I doing:

I have some line--layers and some polygon-layers. I buffer the line-layers with a fixed distance buffer so I get all polygon-layers. Each polygon-layer is converted to lines. When merging all converted layers I get that message.

I tried to eliminate the layers who are responsible for the error. I found two. One was originaly a line-layer that I buffered, the other was originaly a polygon-layer. So I can't figure out if the bug is due to the buffering or not...