Now it works as intended. You can select the materials for which you want to change the prefix of the texturename. The actual name of the texture is not changed. You need to do this with a renaming-tool at the OS level.

thanks to everyone

import c4d
from c4d import gui
import os

#GUI
GROUP = 1000
TEXT = 1001
PREFIX = 1002
BTN_OK = 1010
BTN_CANCEL = 1020

prefix = "placeholder"

class userDialog(gui.GeDialog):
    def CreateLayout(self):
        self.SetTitle("Add Prefix")
        self.GroupBegin(GROUP, c4d.BFH_SCALEFIT, 3,5, title = "Adds a prefix to the texturename")
        self.GroupBorder(c4d.BORDER_GROUP_IN)
        self.GroupBorderSpace(20, 5, 20 , 5)
        self.AddStaticText(TEXT, c4d.BFH_SCALEFIT, name="Prefix")
        self.AddEditText(PREFIX, c4d.BFH_SCALEFIT)
        self.AddButton(BTN_OK, c4d.BFH_SCALE, name="Ok")
        self.AddButton(BTN_CANCEL, c4d.BFH_SCALE, name="Cancel")
        self.GroupEnd()
        return True

    def InitValues(self):
        #initiate the gadgets with values
        self.SetString(PREFIX, "Tex_")
        return True

    def Command(self, id, msg):
        #handle user input
        if id==BTN_CANCEL:
          self.Close()
        elif id==BTN_OK:
          self.TexturPrefix = self.GetString(PREFIX)
          self.Close()
        return True


# Adds a prefix to the texturename
def changeTexture(shader) :


    print (prefix)

    # shader ID
    texturePath = shader[c4d.BITMAPSHADER_FILENAME]
    # split aboslute path
    oldTexturename = os.path.split(texturePath)
    # add prefix
    newTexturename = prefix + oldTexturename[1]

    doc = shader.GetDocument()
    doc.AddUndo(c4d.UNDOTYPE_CHANGE, shader)

    # Assign the new value
    shader[c4d.BITMAPSHADER_FILENAME] = os.path.join(oldTexturename[0], newTexturename)
    shader.Message(c4d.MSG_UPDATE)


# Iterate a hierarchy
def recurse_hierarchy(shader) :
    while shader:
        # Test if the current node is a bitmap shader.
        if shader.CheckType(c4d.Xbitmap) :
            changeTexture(shader)
        recurse_hierarchy(shader.GetDown())
        shader = shader.GetNext()

# Main function
def main() :
    mats = doc.GetActiveMaterials()

    dialog = userDialog()
    dialog.Open(c4d.DLG_TYPE_MODAL, defaultw=200, defaulth=50)

    global prefix
    prefix = dialog.TexturPrefix

    doc.StartUndo()
    # Iterate over selected material
    for mat in mats:
        recurse_hierarchy(mat.GetFirstShader())
    doc.EndUndo()


# Execute main()
if __name__=='__main__':
    main()

Thank you very much, right now i am quite happy how it works. only thing is the dialog box opens for each shader because its in the "changeshader" function wich repeats.
I d' like to have one gui-call at the start in the main function, but if i do that theni cant transport the "prefix" argument to the changeTexture(shader) function.

about the time investement i agree, but for this script was more an out of desperation approach in how cinema4d lacks in organizing textures, then a new grind in passion. the script helps me with a hack, to organize them on OS side, for team projects like for instance unity.

Best
Moko

import c4d
from c4d import gui
import os

#GUI
GROUP = 1000
TEXT = 1001
PREFIX = 1002
BTN_OK = 1010
BTN_CANCEL = 1020

class userDialog(gui.GeDialog):
    def CreateLayout(self):
        self.SetTitle("Add Prefix")
        self.GroupBegin(GROUP, c4d.BFH_SCALEFIT, 3,5, title = "Adds a prefix to the texturename")
        self.GroupBorder(c4d.BORDER_GROUP_IN)
        self.GroupBorderSpace(20, 5, 20 , 5)
        self.AddStaticText(TEXT, c4d.BFH_SCALEFIT, name="Prefix")
        self.AddEditText(PREFIX, c4d.BFH_SCALEFIT)
        self.AddButton(BTN_OK, c4d.BFH_SCALE, name="Ok")
        self.AddButton(BTN_CANCEL, c4d.BFH_SCALE, name="Cancel")
        self.GroupEnd()
        return True

    def InitValues(self):
        #initiate the gadgets with values
        self.SetString(PREFIX, "Tex_")
        return True

    def Command(self, id, msg):
        #handle user input
        if id==BTN_CANCEL:
          self.Close()
        elif id==BTN_OK:
          self.TexturPrefix = self.GetString(PREFIX)
          self.Close()
        return True


# Adds a prefix to the texturename
def changeTexture(shader) :
        
    dialog = userDialog()
    dialog.Open(c4d.DLG_TYPE_MODAL, defaultw=200, defaulth=50)
    
    prefix = dialog.TexturPrefix
    print (prefix)
    
    # shader ID
    texturePath = shader[c4d.BITMAPSHADER_FILENAME]
    # split aboslute path
    oldTexturename = os.path.split(texturePath)
    # add prefix
    newTexturename = prefix + oldTexturename[1]

    doc = shader.GetDocument()
    doc.AddUndo(c4d.UNDOTYPE_CHANGE, shader)

    # Assign the new value
    shader[c4d.BITMAPSHADER_FILENAME] = os.path.join(oldTexturename[0], newTexturename)
    shader.Message(c4d.MSG_UPDATE)


# Iterate a hierarchy
def recurse_hierarchy(shader) :
    while shader:
        # Test if the current node is a bitmap shader.
        if shader.CheckType(c4d.Xbitmap) :
            changeTexture(shader)
        recurse_hierarchy(shader.GetDown())
        shader = shader.GetNext()

# Main function
def main() :
    mats = doc.GetActiveMaterials()
    
    doc.StartUndo()
    # Iterate over selected material
    for mat in mats:
        recurse_hierarchy(mat.GetFirstShader())
    doc.EndUndo()


# Execute main()
if __name__=='__main__':
    main()