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Airlines use the Timatic database to assess what documents a passenger must produce in order to board. There are several public portals to Timatic; this answer used this one.
Germany > Sweden:
Here's Timatic's report for this flight:
Thus, you must show either a passport, Swedish national ID card, or a Swedish termporary passport...and that doc must be valid on arrival. With only an expired Swedish ID card, the airline is likely to refuse to board you.
Sweden > Germany:
For a Swedish citizen, this is almost same. Here's the screenshot:
The difference is that for this flight, no "temporary passport" method is listed.