New installed Verion 1.8.16 problems

I have programs running for years on previous Arduino versions (1.8.13 and earlier).
I installed now the current 1.8.16 and get several errors on compilation:
Repeating tens times on similar lines:

c:\program files (x86)\arduino-1-8-16\hardware\tools\avr\avr\include\stdlib.h:300:21: error: 'size_t' was not declared in this scope extern void *malloc(size_t __size) ATTR_MALLOC;"

Another error that repeats itself for each print and println command:

error: 'class HardwareSerial' has no member named 'print'; did you mean 'Print'?
error: 'class HardwareSerial' has no member named 'println'; did you mean 'Print'?

(Its certainly with an L, not I, and its shown in red, recognized as command.
The Serial has been initialized: Serial.begin(115200);).

the libraries I use (same as for 1.8.13):
#include <Arduino.h>
#include <EEPROM.h>
#include <RTClib.h>
#include <ModbusMaster.h>
My guess is that the version is missing a header file, but this is for experts.

Please help.

It sounds like the core was not installed properly and you're missing multiple files.

You can uninstall and try again; you probably already did try. After the uninstall, I would also delete C:\Users\yourUserName\AppData\Local\Arduino15 (make a backup first).

@samtal Make sure you install all the necessary package core for your board, see