and tnx a lot for your extremly usefull suggestions.
ad 1. > sessionInfo()
R version 3.4.3 (2017-11-30)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows >= 8 x64 (build 9200) [omg - it has changed again!]
Matrix products: default
 LC_COLLATE=German_Germany.1252 LC_CTYPE=German_Germany.1252
 LC_MONETARY=German_Germany.1252 LC_NUMERIC=C
attached base packages:
 stats graphics grDevices utils datasets methods base
other attached packages:
 ggplot2_2.2.1 keras_188.8.131.5202 tensorflow_184.108.40.20600
 reticulate_220.127.116.1100 RevoUtils_10.0.7 RevoUtilsMath_10.0.1
loaded via a namespace (and not attached):
 Rcpp_0.12.14 whisker_0.3-2 magrittr_1.5 munsell_0.4.3
 colorspace_1.3-2 lattice_0.20-35 R6_2.2.2 rlang_0.1.6
 plyr_1.8.4 tools_3.4.3 grid_3.4.3 gtable_0.2.0
 tfruns_1.1 yaml_2.1.16 lazyeval_0.2.1 tibble_1.4.1
 Matrix_1.2-12 base64enc_0.1-3 zeallot_0.0.6 labeling_0.3
 compiler_3.4.3 pillar_1.0.1 scales_0.5.0 jsonlite_1.5
ad 2. Installation: I don't think there is much unusual with my installation. The only 'irregularity' I should mention is that the packages tensorflow, keras & reticulate by some inattention got implanted into the documents section of my HD instead of one of the usual program files. They are working seamlessly, however.
ad 3. Have a look at the following two text examples:
1297 # es genügt dzt. nicht, nur den u.U. sehr kurzen Prognosebereich für Pseq zu
1298 # definieren: wenn nicht mindestens 1 Fall jeder Klasse auftritt, hat proga
1299 # nicht mehr 3 Klassen und das Programm st?rzt [should read: stürzt] ab.
1326 # bzw. ältere (max 20 Rdn) Gewichte bzw.
1327 # beim 1. Mal die Gewichte vom Zeitpunkt der ersten Initialisierung als
1328 # Initialisierung der nÃ¤chsten [should read: nächsten] Runde...
In the meantime I have experimented with these examples in the following way: I copied the text from the original program, shown above, in 2 separate R-files. The first one (A) with encoding ISO-8859-1 (presently indicated as my system default), the second one (B) with UTF-8.
Upon reopening these two files under the unchanged system default setting, (A) showed no change, while (B) showed the initially correct characters ü & ä mutated to the same strange characters implanted earlier into the text.
Apparently, at some time (see remarks below), the default encoding in my system has temporarily changed from a previous state to some other state (perhaps UTF-8, but I don't know either). When working with previously generated program files, the umlaut characters have been converted into some other character combinations; I corrected only part of them and stored the files again; this was repeated several times. Thus, a mixture of erroneous and correct characters was produced. This applies even to "new" files which invariably have been created from parts of copies of old ones, just containing different versions of a program under development. Thus, several files got infected with the character mixture.
The root cause of the problem is still unclear. However, it might be that a complete system crash of my computer following a forced WIN10-update on July, 25th, produced this, as it did with numerous other parameter settings. The system had to be set up completely anew as none of my back-ups could be reanimated, only data files have been recovered. This would explain the 1-month-period since when I observe the reported problems.
I haven't updated RStudio yet but will do that soon - thanks for the reminder anyway!
Note added a few minutes after posting the above:
Reading the last reply (7 May) to the post Unicode replacement character (�) issue in RStudio only within R markdown files, I should perhaps wait with the proposed RStudio update, shouldn't I?-)