Our friend D0gr0ck has found a potential fix to some problems with the Steam client; he has posted it in this thread on the SK Steam forum.
His first post says:
"After some sleuthing around the game files I've found that Steam distributes it's own Java package with Spiral Knights as opposed to using the default Windows installation that the original Three Rings client does. I've found that the Java package from Steam is out of date and could likely be causing many, if not all of the appcrashes reported to far.
The current Java version reads as SE 6 U25 from 2011 in the file properties. Steams is reading as SE 6 U7 from 2004.
You can update the Java files in your Steam game by copying a complete up to date (32 bit only) release into the game folder.
Go to *\Steam\steamapps\common\spiral knights and back up the java_vm foler for just in case and create a new blank one.
Go to C:\Program Files (x86)\Java\jre6 (no x86 on 32 bit Windows) and copy everything inside that folder.
Copy everything into the java_vm folder from Step 1
Run the game like normal.
Note: The new Java version will cause the the Verify Game Cache option to re download the old Java again.
If those having trouble could try this and report back their results it would be very helpful. While I don't seem to have the problems present on my computer a Java update has been a bread and butter fix for many players experiencing problems in the past.
So far this ins't a miracle fix for users, but it does help narrow down the list of options for why only Steam users are experiencing some of these problems."
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Please let us know if this helps you at all.
Unfortunately, from the few people that have tired this so far there's been no success story. Though don't discount it as a potential fix just yet. The out of date Java could still be a factor in a larger problem/oddity with the steam version.