I meant asking IT what they did that may cause this behavior. It is obviously related to the version upgrades and difficult for us to figure out when we don't know the steps taken by IT.
Already asked this: Office 365 is deployed by Microsoft and IT means, we have an absolutely default deployment, without any customization. No chance to work... and we need a second licence, while the first is no longer working 
Can we do a Teamviewer session tomorrow or later this week? I can try and reinstall Seotools on your computer and see if I can find the issue.
Please contact me at victor@seotoolsforexcel.com about this and also write your license email.
/Victor
Wow, a big step further! I realized, that the problem exists only with HTTPS-URLs. With HTTP everything is working like expected.
Maybe this would help to debug things: https://mehdi.me/a-tale-of-debugging-the-linkedin-api-net-and-http-protocol-violations/
That is good news 
Can you ask IT to temporarily turn off any firewalls or other type of security for testing purposes?
No Firewall is the holy cow here...
After STFE update 8.0.86 is the trouble gone. Office updates are stopped, so i hope it works in the future like expected. The cause of the problem is still unknown...
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8.0.90. Nothing works again, with the same error: "The server committed a protocol violation. Section=ResponseHeader Detail=CR must be followed by LF".Tried some machines in different networks, Excel 16x365 and Excel 2010. It is not funny, guys, seriously!
The only difference to previous cases is, that the error appears now not in the popup, as previously, but appears in every table cell (previously it was #VALUE in cells).
Can you contact me at victor@seootolsforexcel.com and I'll send you the latest build which has various improvements to the installer related to Excel permissions.
Thank you, it works again
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STFE 8.1.25: stopped working with the same error - no Office or STFE updates, but the latest Windows updates...
Can you try and uninstall Seotools and install again? I recommend saving a copy of the seotools.config.xml file to keep your settings.
This story lasts nearly half of a year and makes me crazy. Several Office updates passed by, and some STFE updates too. Now STFE works again, without i did anything relevant. And i don't know, whether it will work tomorrow.
It is something beyond of my understanding, and it makes the usage of STFE no longer reliable enough to count on it regarding crucial tasks.
I recommended to install STFE for some corporate clients to make them able to use automated API-based reporting dashboards build on STFE. Maybe sad time has coming to switch to GSheets., i dont know. It reminds me on early 90s, where it was a kind of lottery, whether a software will work or not and one had never known, why.
We have not received any reports of this issue so most likely it is local in nature. It is very strange that it's happening on other machines and networks. I assume Office 365 is the common denominator?
Can you try and install Seotools as a regular add-in from the Add-ins menu? I'll email you the link to a build which is not in msi format.
As i said - today it is not working. Same error, even from spider.
We have not received any reports of this issue so most likely it is local in nature.
I fully understand this - what i not understand, is how a reproducable error could be uncatchable.
No, installing from Add-Ons gived no success - same error again.
The issue is ultimately debugged and solved. Special thanks to @ diskborste for substantial hands on help.
It was a network-caused issue, which had in the first approximation something to do with
- IP address (proxy usage partly avoided the issue) and / or
- usage of Imperva / Incapsula (an affected site was using it).
The last hint, which finally lets STFE work like expected under all circumstances, was to switch from the wireless router to the LAN cable. As stated by the network administrator, wireless router could optimize a data stream on the not always optimal way.
Update: after further investigation we narrowed the issue cause. It was not ther wireless router, but the the WiFi adapter, used by our most machines. It is a Yezala AC1200, which seems to be selled only in Germany. It caused some issues with other (accounting) software.
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