I think the issue is that you have an unnamed argument (pubmed); if you look at the source code, you can see that the first argument should be bdata; if you provide the arguments like you do, it is, however, unclear whether bdata is "pubmed" or the named argument bdata, therefore the error you obtain.
You can reproduce it with this minimal example:
def dummy(a, b):
return a, b
dummy(10, a=3)
will return
TypeError: dummy() got multiple values for argument 'a'
If you remove "pubmed", the error disappears, however, the output is still incomplete:
from bioservices import EUtils
s = EUtils()
print(s.ECitMatch("proc+natl+acad+sci+u+s+a|1991|88|3248|mann+bj|Art1|%0Dscience|1987|235|182|palmenberg+ac|Art2|"))
returns
'proc+natl+acad+sci+u+s+a|1991|88|3248|mann+bj|Art1|2014248\n'
so only the first publication is taken into account. You can get the results for both by using the correct carriage return character \r:
print(s.ECitMatch(bdata="proc+natl+acad+sci+u+s+a|1991|88|3248|mann+bj|Art1|\rscience|1987|235|182|palmenberg+ac|Art2|"))
will return
proc+natl+acad+sci+u+s+a|1991|88|3248|mann+bj|Art1|2014248
science|1987|235|182|palmenberg+ac|Art2|3026048
I think you neither have to specify retmod nor the database (pubmed); if you look at the source code I linked above you can see:
query = "ecitmatch.cgi?db=pubmed&retmode=xml"
so seems it always uses pubmed and xml.