Which jPCT version is that? 1.15? 1.16beta? I don't think that i have changed something related to texture management and if it works with an older FenGUI, that's most likely not the problem anyway...but maybe i'm doing something wrong that has never hurt the old version of FenGUI but does hurt the new one. Then again, uploading textures is nothing special. There's not much room to do it wrong.
However, some wild suggestions to try:
Disable mipmapping and trilinear filtering if used via the appropriate settings in Config.
Try to set Config.glTextureDepth to -1
Try to set Config.useFBO to false
Try to set Config.glOverrideStageCount to 1
Try to set Config.glForceFinish to true
I don't think that any of these settings will change the outcome, but just to be sure...