I'm testing out Boinc 7.16.7 (x64) on Windows 10 home (64 bit).
It all works fine, however, when I tried limiting a project to 2 simultaneous WUs in app_config, Boinc doesn't honor the values.
In the log there's a note saying 'Missing start tag in cc_config.xml'.
Thinking that cc_config.xml needs to be set before app_config.xml can be set, I populated it with this:

<cc_config>
<options>
</options>
</cc_config>

As I'm not needing any special configurations there.
But I still get the error.
Instead of telling me 'name is wrong in app_config.xml' or so, it tells me there's something wrong with cc_config.xml?

No, you don't need a cc_config.xml for using an app_config.xml

So please post the contents of your app_config.xml and do tell how you edited it, and what format you saved it in (should be ANSI).

---

I used nano. It usually works fine.

app_config.xml content:

<app_config>
<app>
<name>mlds</name>
<max_concurrent>1</max_concurrent>
</app>
</app_config>

Why not paste the contents into Notepad, see if there's nothing weird going on there and then save it via there as ANSI?
You may have extra (invisible) linefeed characters. You may have saved it as UTF-8. Both of these will break the parsing of the XML file.

---

Unless Windows has a similarly named editor, nano is the standard command line editor of Linux. So shouldn't have any issues with formatting.

It appears to affect only the Windows version.
Linux seems to work fine with it.

I've used Notepad++ to see if any strange characters were present, but nothing so far.
Besides, it would be odd for Boinc to refer to cc_config, when there's an error in app_config.
The default cc_config was blank, I might revert it to a blank file again; and for the mean time, adjusted my CPU settings to 50 or 75%, as MLC is the only project currently running (next to collatz).

Strange character - that indicates you have not save the file in the correct format. Notpad++ can save in quite a number of character formats, make sure you are using ANSI encoding. This has tripped up quite a few in the past so don't be ashamed, just join the club.

You have the error because for a sanity check the cc_config parser is used. That one only has an error message about cc_config.xml

I've two times said to save in ANSI format

I've requested that app_config.xml (and possibly other XML files) get their own integrity and sanity check in #3938

Interesting. Fortunately, I have plenty of machines with working app_config.xml files for SETI@Home, currently unused.

So I saved one in UTF format, and re-read all config files. Result?

22/07/2020 14:32:52 |  | Re-reading cc_config.xml
22/07/2020 14:32:52 |  | Config: allow multiple clients
22/07/2020 14:32:52 |  | Config: GUI RPCs allowed from:
22/07/2020 14:32:52 | Einstein@Home | Config: excluded GPU.  Type: NVIDIA.  App: einstein_O2MDF.  Device: 1
22/07/2020 14:32:52 |  | Config: use all coprocessors
22/07/2020 14:32:52 |  | log flags: file_xfer, sched_ops, task, cpu_sched, sched_op_debug
22/07/2020 14:32:52 | Einstein@Home | Found app_config.xml
22/07/2020 14:32:52 | GPUGRID | Found app_config.xml
22/07/2020 14:32:52 | SETI@home | Found app_config.xml
22/07/2020 14:32:52 | SETI@home | Missing <app_config> in app_config.xml

So, what am I doing differently?

Renamed cc_config.xml so it wouldn't get in the way.

22/07/2020 14:40:10 |  | Re-reading cc_config.xml
22/07/2020 14:40:10 |  | cc_config.xml not found - using defaults
22/07/2020 14:40:10 |  | log flags: file_xfer, sched_ops, task
22/07/2020 14:40:10 | Einstein@Home | Found app_config.xml
22/07/2020 14:40:10 | GPUGRID | Found app_config.xml
22/07/2020 14:40:10 | SETI@home | Found app_config.xml
22/07/2020 14:40:10 | SETI@home | Missing <app_config> in app_config.xml

Re-saving the SETI app_config.xml file in ANSI format, using Notepad, resolved the error.

Windows 10, 2004, Notepad:

UTF-8:
22/07/2020 16:49:18 | SETI@home | Found app_info.xml; using anonymous platform

UTF-8 with BOM:
22/07/2020 16:50:17 | SETI@home | Found app_info.xml; using anonymous platform
22/07/2020 16:50:17 | SETI@home | Unparsed line in app_info.xml: 
(but no mention what the line is)

UTF-16LE:
22/07/2020 16:47:33 | SETI@home | Found app_info.xml; using anonymous platform
22/07/2020 16:47:33 | SETI@home |
22/07/2020 16:47:33 | SETI@home | Syntax error in app_info.xml

UTF-16BE:
22/07/2020 16:51:34 | SETI@home | Found app_info.xml; using anonymous platform
22/07/2020 16:51:34 | SETI@home | Syntax error in app_info.xml

ANSI:
22/07/2020 16:52:24 | SETI@home | Found app_info.xml; using anonymous platform

So for completeness, I made an app_config.xml, saved it as UTF-8 from Notepad in Windows 10, 2004.
Added into it:

<app_config>
<app>
<name>kaktwoos</name>
<max_concurrent>1</max_concurrent>
</app>
</app_config>

And saved it into the Minecraft directory.

Then started BOINC:

22/07/2020 16:57:52 | minecrafthome | Found app_config.xml
22/07/2020 16:57:52 | minecrafthome | kaktwoos: Max 1 concurrent jobs

So I can't reproduce it either, unless I have to install Nano and I am not going to do that.

My test was using the Notepad from Windows 7. I'll try again since I got an error and you didn't, and poke at the resulting file with a hex editor to see exactly what damage UTF-8 does.

Under Windows 7, the UTF-8 file seems to be three bytes longer than the ANSI file. The extra three bytes come at the beginning, and are EF BB BF. Now to try the same thing with Windows 10 2004...

And under Windows 10 2004 (the same build 19041.388 I was using yesterday), the UTF-8 file is the same length as the ANSI file, and there are no visible differences. Adding the three extra bytes, Notepad now recognises it as 'UTF-8 with BOM', and BOINC can't find <app_config> in app_config.xml.

I think that concludes the examination of Windows file types, but still doesn't explain the OP's error message for the wrong file.

Ok, the issue disappeared by deleting cc_config.xml.

It's really odd, because I haven't touched that file, when the error happened.
It happened after adding MLC@Home, and modifying it's app_config.xml file.

I haven't adjusted app_config.xml, just deleted cc_config.xml, since I don't have anything useful in there anyway, and that did the trick.