After many more support correspondence with SoftwareFX technical support, yesterday I received an update confirming that the problem had now been reproduced. I quote:
"We have been creating different samples from scratch and the error was finally replicated on an xp machine with a new sample project we created when using exactly the following configuration: forms authentication, SqlMembershipProvider and .Net Framework 2.0 SP2. The error does not occured if any of the above-mentioned configuration is missing."
I re-iterated that I re-produced the problem myself on severs with .NET 2.0 RTM and .NET 2.0 SP2, either in combination with client with .NET 2.0 SP1 (on XP), .NET 2.0 SP2 (on XP) and .NET 3.5 (on Vista) respectively in the hope that any fix to the problem will be tested thoroughly including these configurations too.
As to the resolution, I quote:
"Although this is a rare situation, there are particular scenarios (as in this case) in which an unexpected behavior may be experienced. I have submitted this problem to our development team for analysis. Please note that it is going to be attended according to the severity of the problem and the priority assigned by our development team. In the case of this being confirmed as a bug, please note that the timeframe for bug resolutions will vary depending on the severity of the bug. Once a bug is fixed, it is usually made available as part of the next service pack."
I post this reply as I hope nobody else has to go through the frustrating experience I had. On the technical side, just to clarify, I encountered the problem initially with ChartFX 7 binaries from builds dated Nov. or Dec. 2008 and applying the Jan. 2009 service pack didn't make any difference.
I will update this when it is resolved, unless of course, somebody from SoftwareFX beat me to it.