I was thinking prime tower also. If the nozzle leaks a bit hopefully not enough to fall into the print before you wipe that leakage off on the prime tower.
Also you might want to print the tpu slower than normal such that the pressure in the nozzle is lower (hence hopefully less leaking) and you might want to increase your retraction a lot. Watch the tpu in the bowden. You want it to rest on the bottom of the bowden arch when retracted. You might need to go as high as 10mm or even higher for retraction distance if the tpu is bunching up a bit.
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Brulti 177
TPU and PVA are listed as experimental by Ultimaker, thus you may encounter other problems.
One solution to your problem would be a prime tower, whihc would gives TPU time to cool down and stop extruding before PVA starts printing the support of your model.
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