This topic was included in Hardware FAQ by @Gute_Laune in the meanwhile: 
Edit: Translation by deepl.com
- The TonUINO sometimes behaves strangely and I used additional flux.
- If you have used soldering grease, you have to wash the board thoroughly with hot water (best with a little rinse). Then bathe in alcohol or methylated spirits and dry well. The reason for all this is that soldering grease contains acid, which creates creepage distances that can impair or even completely prevent the function of sensitive electronics. Please remove the electronics before!
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