Remember to cover the basics, that is, what you expected to happen and
what in fact did happen.  You don't know how to make a good report?  See

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------------------------------------------------------------------------

Hi,

ob-plantuml wraps a plantuml source in @startuml/@enduml tags without
checking whether they are already in the source code. This results in
the creation of invalid SVG, at least for newer versions of plantuml,
because the resulting SVG file contains then two XML documents ((I was
using version 1.2018.2, it seems to work though for older plantuml
versions <= 8059).

So the following does not work

#+BEGIN_SRC plantuml :file "out.svg"
@startuml
a->b
@enduml
#+END_SRC

while this works:

#+BEGIN_SRC plantuml :file "out.svg"
a->b
#+END_SRC

Every plantuml example I found contains @startuml/@enduml so all my old
org mode files do contain the tags... I feel the required change is not
very difficult, but unfortunately I do not speak elisp at all...

Emacs  : GNU Emacs 25.2.2 (x86_64-pc-linux-gnu, GTK+ Version 3.22.28)
 of 2018-02-19, modified by Debian
Package: Org mode version 9.1.7 (9.1.7-20-ge1f1ac-elpa @ /home/guertler/.emacs.d/elpa/org-20180319/)

Thanks for considering,

Martin

-- 
Dr. Martin Gürtler