inverse synctex not working / SyncTeX ERROR: SyncTeX Error: Bad preamble

Hi all,
I regularly using Sweave for report generation inside RStudio (Tiger Daylily) with R 4.0.5, TexLive 2015, SumatraPDF on Win7 (migration to Win10, didn't change anything).
Unfortunately the synctex feature is not working properly.

A simple .Rnw file as
\documentclass{article}

\usepackage{Sweave}
\begin{document}
\SweaveOpts{concordance=TRUE}

test 1

\newpage

test 2

\end{document}

does compile (using Compile PDF button in RStudio), but syncing does not properly work:

  • compiling the pdf, all necessary files are available: .log, .pdf, .tex, -concordance.tex, .synctex.gz, but
  • R-console shows: SyncTeX ERROR: SyncTeX Error: Bad preamble (only on 32bit-R)
  • in 32bit-R the pdf does not show up automatically
  • in 32bit-R sync-pdf button only appears only if the file is touched again (typing, delete, comment, uncomment,...) and only THEN the following is TRUE (before, syncing does not work, on 32bit-R pdf does not even show up although available)
    ** syncing from .rnw to .pdf does work correctly (ctrl-click)
    ** syncing from .pdf to .rnw does not work properly, as cursor does not jump to the required position - but stays at its position (double-click/ctrl-click)
  • in 64bit-R the generated pdf show up imidiately, but no sync-pdf button either until touching the file
  • after touching the file, sync-pdf button appears and
    ** syncing from .rnw to .pdf does work correctly (ctrl-click)
    ** syncing from .pdf to .rnw does not work properly, as cursor does not jump to the required position - but stays at its position (double-click/ctrl-click)

I tried to understand the 'bad preamble' error, but failed. There is little (helpful) information about that on the web.

Meanwhile, it seems to be a problem in conjunction with SumatraPDF, because using 'RStudio Viewer' on 64bit-R everything works fine (but it gets even worse on 32bit: each ctrl-click leads to a Bad preamble error and no syncing at all, not even using a .tex file - sth which works perfectly using texWorks ).

Anyone an idea?

Thank you very much!
C.