I can't reproduce this - here's what I did:

PxPlus-2019 Web (Ver:16.10/UNIX-Linux-RedHat) Serno:1610-664-0xxxxxx
(c) Copyright 2005-2018 PVX Plus Technologies Ltd. (All rights reserved)
  Website: http://www.pvxplus.com
->010 BEGIN
-:0020 LET BASE64_FILE$="/tmp/50_09790301_13_200_002_led_kalender.base64"
-:0030 LET PDFFIL$="/tmp/50_09790301_13_200_002_led_kalender.pdf"
-:0040 OPEN (1)"*PDF*;FILE="+PDFFIL$+";OVERWRITE"; LIST (1); CLOSE (1)
-:0050 ERASE BASE64_FILE$,ERR=*NEXT                                   
-:0060 CALL "*web/base64;ENCODE_FILE",PDFFIL$,BASE64_FILE$           
-:run                                                                 .                     
-:open input(1)base64_file$;print rcd(1)
JVBERi0xLjMKJbe+raoKMSAwIG9iago8PAovVHlwZSAvQ2F0YWxvZwovUGFnZXMgMiAwIFIKPj4KZW5kb2JqCjIgMCBvYmoKPDwKL1R5cGUgL1BhZ2VzCi9LaWRzIFsgNSAwIFIgXQovQ291bnQgMQo+PgplbmRvYmoKMyAwIG9iago8PAovUHJvZHVjZXIgKHd3dy5wdnhwbHVzLmNvbSkKL0F1dGhvciAocm9vdCkKL0NyZWF0b3IgKFB4UGx1cy0xNi4xMC4wMDAwKQovVGl0bGUgKFVudGl0bGVkKQovU3ViamVjdCAoTm8gU3ViamVjdCkKL0tleXdvcmRzIChub25lKQovQ3JlYXRpb25EYXRlIChEOjIwMjIwODI0
-:

I open the base64 file in gedit, copy the contents and paste them on that website.  The result is the attached picture (the correct PDF contents).

Is this close enough:

0010 BEGIN ; PRINT 'MODE'($000F$),'CS',
0020 PRINT 'OPTION'("GridMLVersion","0"),
0030 LET G=10; GRID G,@(38,1,40,8)
0040 LET G'COLUMNSWIDE=4,G'ROWSHIGH=5,G'ENTERMODE=2
0050 LET G'COLUMN=0,G'ROW=0,G'COLUMNWIDTH=8,G'CELLTYPE$="Multi_line"
0060 FOR R=1 TO 5; FOR C=1 TO 4; LET G'COLUMN=C,G'ROW=R
0070 LET G'VALUE$="C"+STR(C)+"/R"+STR(R); IF R/C=1 THEN LET G'LOCK=1
0080 NEXT ; NEXT ; LET G'CURRENTCOLUMN=2,G'ROW=1; SET_FOCUS G
0090 WAIT:OBTAIN *; IF CTL=4 THEN STOP
0100 GOTO WAIT

<enter> stops editing the cell, and the next <enter> moves focus to the next cell.

When you get that message during a PROCESS, the message comes from the GET_VERSIONS routine in *WINPROC:

14320 LET _X$=MSE
14325 IF MID(_X$,1,1)=$FF$ THEN PRINT "Not a graphical device -- cannot use Nomads panels"; EXIT

Can you enter these commands in PxPlus console mode and show us the output:

PRINT CVS(FIN(0),16)
PRINT HTA(MSE)

I think the trick is to create a 'Calendar Control Definition' that doesn't use any separation characters, like 'YYYYMMDD' or 'MMDDYYYY' (instead of 'YYYY/MM/DD' and 'MM/DD/YYYY').

In the multiline that receives the date, you want to use an 'Input Mask' with separation characters (0000/00/00 or 00/00/0000 in my examples).

And the 'Initial Value' should be a true date, for example Expression DTE(0:"%Yl%Mz%Dz") or Expression DTE(0:"%Mz%Dz%Yl") - again without any separation characters. 

I've attached an example. Import 'calendar.txt' in a Nomads library.  Create the Calendar Control Definitions (all three) like shown in the pictures (same names and properties).

Should work in v15 up to v18.

Call "*cmd/system/loadbbx","name_of_bbx_program"

will transfer a BBx program to PxPlus program memory and

Call "*cmd/system/clip","1-64999"

will transfer the PxPlus program to the clipboard.  Both commands also work from console mode:

->loadbbx bbx_program
-:clip 1-64999

Can you check if you have a *plus/dll/pluslib_base.dll file (on the WindX side) ?  If not, you need to download the v17.10 PxPlus Windows ZIP download, extract that file and copy it to your Windx lib/_plus/dll folder.  Then restart WindX and try again.

Can you add these commands to your start program:

SET_PARAM 'TU'=0; LET  %NOMAD_Turbo_Off=1

Regarding the missing HTML - if you installed v17.10 on top of a previous version, you may want to repair the v17 installation (first stop all processes/services that are running v17).