After a short time of working, the font styles disappear from the font menu. They are then no longer selectable.
If I select Text in the document, the Myriad is displayed instead of the correct font.

I have checked which actions the problem occurs. Even if there is no user action and the document has been opened correctly and without error, Illustrator will switch to Myriad after a few minutes and will partially remove the fonts of the previously selected font from the menu.

I took the following test:
I have opened a document. ClanOT was previously selected as the font. Illustrator does not yet show any error. Then I didn't touch the computer and program for about five to ten minutes. When I wanted to continue working from the document, several font cuts had disappeared from the menu and the Myriad was set.

The error occurs in Illustrator without any action by the user.
I have deleted the font caches several times. The problem remains.

I apologize if I misrepresented myself. My English is not perfect, so I use an online translator.

What I mean is the following:
I have created a document, selected a font and written text. Font is ClanOT bold. The typeface also has the cuts "book", "book Italic" and "bold Italic". After a few minutes I can only select bold. Or just Italic, for example. All other sections are no longer visible in the menu.
The text in the document is highlighted, but then often Myriad is shown in the menu. (Not in the picture below)
However, the text itself is still displayed in Clan.

The font is not damaged. It works in all other programs.
The problem affects not only the font Clan but some others as well.
My font management program is compatible.

Please have a look here: https://forums.adobe.com/message/10160237#10160237

Looses bitmap figure embedded in .ai file, reports the bitmap is missing when opening. While other software opens the file just right. Including Adobe Acrobat, Gmail preview, apple finder preview and Affinity Designer.

I made a file using Adobe illustrator, this file contains two jpg images. When I save the file and open it using illustrator on two different computers, a dialoge appears complaining the jpgs are missing and offer to replace them.
Oddly other software packages read the same file just fine, INCLUDING the allegedly missing jpg images, and in good quality.

I can provide the file if you request.

I go to print a file. Select my printer preset.
I get the error
"The specified PPD is invalid. The default PPD will be used."
I hit OK.
Print window pops back up
I manually select the printer. Hit print. I get another error that says" can't print file"
I hit OK
File -print again.
I manually select the printer and the ppd to match.
Make sure everything else. Size, placement of the file are okay. Hit Print. Then it will print.

I tried resaving my preset, but this has not changed the issue.

It seems like as time has gone on this error has gotten worse.
Months ago it didn't use to do this. Then it would jump off of my preset. Now it's evolved to this.
Our printer is a XEROX 550-560 Integrated Fiery

Consistent repeat crashes. I'm on a MAC High Sierra. I know my Illustrator 21.1 will at least crash around 10am and 3pm EVERYDAY. It goes through phases where it will crash 4 times a day.

I am merely opening an Illustrator file from my desktop or in a folder on my desktop that is 1mb -5mb most times when this happens. When clicking on the file and waiting for this small file to open and nothing happens I realize that it's locked up.

I check the task manager and sure enough it's not responding and I have to force quit the program. It was so bad I was keeping a journal. Then it got so ridiculous that I quit. It's how I know what time of the day it is.

When opening an .ai file on one particular PC in our workplace (a file that was created on a different PC) most of the vector shapes show as black-filled shapes.

We have multiple copies of Illustrator (all version 22.0.1 (64-bit) in our workplace and on every other PC the artwork displays properly (with no black shapes).
Turning on 'Overprint Preview' on the troublesome PC fixed the issue.
On every other PC turning this option on or off has no effect.
If we create a new file in Illustrator on this troublesome PC then we don't experience any problems?
I have attached two screen grabs (one from the troublesome PC and one from a different PC) which show the issue.
This one particular PC had a Windows 10 update just yesterday (26 Feb 2018).

I am using Illustrator 22.0.1 to design a logo. I've tried this on both my MacBook Pro and on my iMac and had the same thing happened:

With the document open, I double-clicked on of the words to get the Type panel to open on the right. I then made my selection with the Type tool and then clicked the font drop down menu to change the font and started scrolling.

I need to be able to scroll through the font dropdown menu to browse the fonts so I can make a selection.

Instead, the font menu bugs out and automatically closes, and then changes the type I have highlighted to some random font.