Soluble Material checkbox ignored when printing with wipe tower

[ungutknut]

Description of the bug

On a 2 toolhead Prusa XL I'm trying to print a project with soluble supports on prusa slicer 2.9.3.

Extruder 1 is PETG, Extruder 2 is BVOH (checkbox "Soluble material" is enabled).

When I try to enable a wipe tower, an error message appears:

This makes absolutely no sense to me. I didn't select non-soluble supports; it's soluble. Also "support_material_extruder" and "support_material_interface_extruder" are set to extruder #2 on purpose. If I set them to 0, the print is sliced with no soluble support at all.

Since one of the main advantages of a 2-toolhead system obviously is to print with soluble supports, this message is an absolute bummer and probably makes a lot of users instantly regret buying a Prusa XL. So fixing this should have a high priority.

Fortunately there's a workaround: You have to set "Top contact Z distance" to "0 (soluble)"... but I had to search some forum posts on the internet for it instead of an error message giving a hint to what to do. Also I'd prefer to choose an arbitrary top contact z distance.

Then another unexpected (minor) limitation appears: I can only use this setting when "Synchronize with object layers" is enabled. Which can have a negative impact on some geometry since large bottom surfaces get segmented, creating a predeterminded breaking point.

Project file & How to reproduce

1. Open a .stl that requires support
2. Load extruder 1 with PETG and extruder 2 with BVOH (in this case Fiberlogy)
3. Make sure that "soluble material" checkbox for BVOH is enabled
4. Set "support material/raft/skirt extruder" and "support material/raft/interface extruder" to 2
5. Enable wipe tower
6. A nonsensical error message will appear

Checklist of files included above

• Project file
• Screenshot

Version of PrusaSlicer

2.9.3

Operating system

Windows 11

Printer model

Prusa XL with 2 extruders