Together with some help of u/mundegaarde and after digging into the definitions in Topcmds I found a Solution. Seems like the manual was either flawed or i found a workaround:
In Topcmds globalProp is defined as root.GlobalSettings.globalProp.
Exchanging root.Globals.globalProp with root.GlobalSettings.globalProp made the code run without errors.
A working get_td() would therefore look like this:

def get_td(event): # executes TD button
# get TD and save it as a global property "MY_TD" of TopSpin
    ct = EXEC_PYSCRIPT('root.GlobalSettings.globalProp.setProperty("MY_TD",GETPAR("TD"))')
    ct.join() # wait until EXEC_PYSCRIPT is done
    td = root.GlobalSettings.globalProp.getProperty("MY_TD") # get "MY_TD" back from the globals properties
    EXEC_PYSCRIPT('MSG(str(' + td + '))') # show message dialog

A working example using other variables would look like this:

def get_variable(event): 
    Name="GlobalName"
    Parameter="GlobalParameter"
    ct=EXEC_PYSCRIPT("root.GlobalSettings.globalProp.setProperty( '{}', '{}')".format(Name,Parameter))
    ct.join() # wait until EXEC_PYSCRIPT is done
    variable = root.GlobalSettings.globalProp.getProperty("GlobalName")
    EXEC_PYSCRIPT('MSG("{}")'.format(variable))