Flashplayer stopped working - no EM - Firefox -Win7 32 bit

Hi,

I really need an expert opinion as I am at wits end.

1. before it happened

2. what it looks like

3. specs

4. what I did to solve the issue

1.

I got a "new" pc at work (had previous owners before) and installed new FP (20.0.0.306) for Firefox (44.0.2), as well as unity mediaplayer. Flashplayer worked fine but UMP crashed after 3 mins or so. Tried to solve the issue through a variety of ways (eg reinstall UMP, reinstall FP, bringing graphic drivers up to date, disabling MS malware detection, ....) Finally found a way by writing an older version number into the info.plist.

NOW everything worked fine for the next 30 or 40 minutes until suddenly the FP stopped working.

2.

grey window where the FP should appear (in case of some games I can see a background pic instead)- no error message, no hints, no chance to get the options for this site. The FP is installed and I can access the option via the control panel

If i install FP for IE or Chrome it works perfectly fine.

3.

Win 7    32 bit

FP (20.0.0.306)

Firefox (44.0.2)

4.

I tried quite a few things without being able to solve the issue:

First thing I did was to redo all setting changes I made earlier (except for MS malware detection)

-multiple times reinstalled FP & FF & Java & UMP

-installed FP version 20.0.0.235 but it immediately updated

-dis- and reenabled FP & Java plugins

-deleted Java, FF and FP cache

-reset FF

-dis-and reeenabled java contents in browser

-disabling active x filtering in the IE in all 4 settings (internet, intranet,..)

Any help would be appreciated.